I believe that this is possible as a stacked instrument in the main GS3 application. I haven't tried this in the Editor, so I'm not sure about that approach.
For instance, I often stack the GS3-VSL brass ensemble instruments by combining the sustain-mod_wheel with the staccato. That allows me to control the attack with velocity and the sustain with the mod-wheel. It is only one additional step to make this stack select the instrument with a keyswitch.
That's definitely possible in the deitor, as well. Just find the velocity-sensitive patch and the crossfade patch within the gig file, highlight both, right-click and select, "combine instruments". Select "Keyboard" from the drop-down list of controllers. After clicking ok, you'll see a purple region on the keyboard. Drag its location and and width to the desired keys. Save your file, and you should have a single instrument that you can load and use for both playing styles.
NOTE: If the two instruments you want to combine each have their own iMIDI rules, they are best combined in the channel-stackng method described above by Jon. In fact, his way is much more intuitive and hassle free in general. Just thought I'd play devil's advocate.