yes you can retexture a grunt very easily... you just need to do the following: make the custom mat file... whit a new name... then copy the original grunt's dae file and change its name to the same name you used for the mat...then copy the original grunt's ent file and once again give it the same name you used before... now you just need to open up the ent file whit notpad or notpad++ and rewrite what dae file it will load (the renamed dae file) then edit what mat file it will load (your new mat file for the grunt) save it.... open model editor it should show the grunt's new texture... i hope i said it correctly
hope it helps!