Can you assign pitch bend to the expression pedal?  I am often busy on the keyboard with both hands but need to use pitch bend at times as well eg with guitar sounds or some pop synth sounds.

Technically speaking, no, you cannot. Pitch Bend (En in hexidecimal) is its own category of MIDI message which is sent by the dedicated PB Wheel.

What you can do is assign Coarse Tune or Fine Tune to other controllers… while this will change the pitch, it is not (technically speaking) actually Pitch Bend (En).

There are a couple of reasons why altering the tuning with Coarse Tune via a Foot Pedal may be problematic:

1) you can only execute the bend in one direction, not up and/or down as with the PB wheel (a spring loaded wheel control capable of movement up or down with a center return position)

2) once you select the direction of bend (say you want to bend the pitch in an upward direction) the foot pedal must be kept in the heel down position to facilitate normal pitch. You must move the pedal toward toe down in order to execute the bend and manually return it to heel down to again play normally.

3) the foot is a rather clumsy limb (compared to the hand) when it comes to controlling pitch manually. The lack of a spring-loaded automatically returning-to-zero control makes this difficult to master

4) the Coarse Tune and Fine Tune parameter are not going to align to any particular semitone increments. In fact, when setting up the Control Set and targeting a DESTINATION - Coarse Tune, a +16 setting will equal 1 Octave of pitch change. Trying to set the pitch bend to any exact semitone distance is problematic.

If you wish to explore this (anyway).
Call up the VOICE
Press [EDIT]
Press [COMMON EDIT]
Press [F4] CTRL SET
SOURCE = FootCtrl2(04)
DESTINATION = Coarse Tune
DEPTH = to your taste
ELEMENT SWITCH - mark the Elements you want to adjust 1-8

You might want to explore Aftertouch as a possible alternative controller. The Aftertouch message (Dn in hexadecimal) is, again, its own classification of MIDI control message. You apply additional downward pressure on the keyboard to apply the “Coarse Tune”. Because it is controlled with your hand and because it is going to automatically return to 0 when you ease off, it is far more intuitive than the Foot Controller (when it comes to controlling pitch change. You do have a similar limitation in that the pitch change will only be available in one direction.

Hope that helps.

Another detalied excellent prompt reply for which I am most grateful. 
When I bought my first synthesizer in 1986 an Ensoniq ESQ1 you just got a manual from a far away country which was near unintelligible and were left to your own devices!

Personally, when I know I won’t be able to use the pitch bend wheel, I like to use the expression pedal to control Coarse Tune and Fine Tune (as Bad Mister described) to get a “bend” effect.
Set a side the fact that Aftertouch stopped working on my Motif ES :-), I think the expression pedal allows more control of the bend itself.
With Aftertouch, the “on” and “off” positions are not very far from eachother, making the range of values in between practically almost uncontrollable. Plus, with Aftertouch you have to use force, which sometimes makes it hard to apply subtle motion - sometimes important in pitch bend.

If your aftertouch stopped working - this precludes any subtle motions to control it, I would guess. You should have it looked at - probably a connector that has come loose.

I don’t think I’m gonna bring it to the labs since they take loads of money just to take the Motif apart.
Anyway, when it worked I didn’t use it for pitch bend either, or to any continuous effects for that matter. Maybe for on/off effects.

Anyway, when it worked I didn’t use it for pitch bend either, or to any continuous effects for that matter. Maybe for on/off effects.

Well then, no wonder you think it is broken. Seriously, you cannot assign Effect ON/OFF to Aftertouch with any kind of great results, here’s why:

Aftertouch is a the definitive momentary control. It is not a switch. In other words, if you assigned it to a function that was responsible for setting an effect to be applied, it would only be applied while you applied Aftertouch - as soon as you let go the effect would shut off. This is what we call a ‘poor choice’ for controller assignment.

Please check out the SUPPORT article on Working with and Understanding Physical Controllers

Sorry, I still think Aftertouch should not be used to control a continuous range, in case some amount of precision is required.
It is better to use the Aftertouch activate an effect (and what I mean by “activate” is change the value from 0 to a non-zero value, for example raise the cutoff value by a certain amount), but you really won’t be able to “fine-control” the effect.
For example, if the depth of the effect using the Aftertouch is 60, then it would be quite difficult to be able control whether the value is at 30 or 40, which can be very important when we’re talking about things like pitch bend. Plus, it would be difficult to control the speed of the bend, especially when required to do so while playing a melody etc.
I do agree that Aftertouch is a momentary control like the bend wheel, but I don’t think this property alone makes it suitable to control a bend.

I feel that my choice of controllers is actually quite good, and often “non-conformist” thinking (i.e. not using controllers exactly according to the manual) can really come into hand.

You misunderstood a few of the issues I explained. Please try to understand the idea I’m trying to convey, rather than trying to reach various conclusions (e.g. I don’t know how to use aftertouch), which feels a bit condescending.
Anyway, it’s kind of you to refer me to a manual, but no manual can replace my experience with controllers.