I am new to workstations, and just wanted to know if this was possible with any of the Motif models...?

Yes, you can sample other keyboards just as you can sample any other audio source.

It’s never the same as actually using the other keyboard but if you have certain favorite sounds you just can’t do without, it can be done.

Here’s how I sample other keyboards. I run my Yamaha SY85 through a mixer to boost the signal. (Some keyboard output signals may not be strong enough to get an adequate recording level in your instrument’s sampler. Maybe yours does if it has a preamp.) I take the main outputs of my mixer into the A/D inputs of my Motif 6. Make sure you turn off your mixer’s volume sliders for the instrument you’re recording into, or you will get heck of feedback!!
I don’t know what instrument you have, so I’m using my Motif 6 as an example. In the INTEGRATED SAMPLING mode, when you set up the Source Type, Source, etc., you have a choice of choosing mono or stereo recording. If you want mono recording, make sure “L+RMono” is selected to the right of the “Mono/Stereo” Obviously, if you want to record a stereo sample, “Stereo” must be selected, and both Left/Right stereo signals must be connected from your mixer (or source) to the corresponding Left/Right A/D inputs on your unit. Then press the REC button. This will take you to the actual record window. From here you select what type of recording trigger you want, how loud you want the monitor to be (this does NOT affect the volume of the sound you’re sampling - only the monitor volume). Your manual should take it from there.
Keep in mind (I’m not positive about this) that stereo samples take up more memory than mono samples. Also, if you like, make sure to turn off the reverb on your original source, and use your synth’s reverb. This will give you more flexibility in how much reverb you want to assign each individual sample/waveform in Voice or Song Mode. Same thing goes for Chorus effect, which intensity can be individually set on each track of a song.