I assume you want to copy a original glest unit and add it as a new one. Thats no problem.
Second: You want to change the original model. This is easy if you just exchange the texture, if you want to change the 3d-model you've got some problems:
The g3d format only saves the model in some keyframes and Glest morphes the vertices between this poses. Since you have no skeleton and new vertices or triangle can hardly be added.
To my mind you've got two possibilities: You could get the original files from wich the g3d was exported. There are some gived for download, but they are in *.max fileformat.
Or you use the first frame of idle animation and build a new skeleton into the model, but thats hard work.
If you want, we can give you a blenderprojekt with one of our viking-models. They have a skeleton, but the mesh must be rigged once again if you change it.
