Is there any way to merge multiple models in a single project without having to lose the weighting or enveloping?

I think this GATOR feature was disabled in Mod Tool, but you can likely do this in the student versions of Softimage.

Edit
You could probably write a script that could do this for Mod Tool.

I'm still learning Mod Tool so scripting is a ways off for me. Never mind, I'll make do with the hard way for the moment.

I performed several tests in Mod Tool 7.5 with both null and bone chain skeletons and it actually works, I recall this being disabled in 6.x.
I broke the unit_template into 3 parts by separating the head and right hand into different parts from the body, then enveloped those parts to the head and hand bones respectively. I merged all 3 parts with weights and materials then deleted the originals and it retained the correct weighting.
I'd say make sure you're using 7.5 and download it from Moddb.com just in case there is a broken version being offered elsewhere and try just merging 2 objects at a time.

It works! Thanks!