You will want the whole MODL chunk.
The trickest part I see is making sure the MNDX fields don't conflict.
If you have to change any, change the added collisions. If you change the MNDX's of existing enveloped bones, you will need to change all the ENVL's in the model that use them.
I have a program in my msh tool kit that can help visualize the layout
pre-alpha_msh_kit.zip
The program is
SWBF_MSH_INFO.exe
That link has the latest version of it.
Also, the TagSizeValidator.exe will load a msh, check every chunk and tag size field and correct it if needed. Saving the result as "tst_inf_tst.msh" so it doesn't overwrite the source file.
(Thats probably the single most usefull tool in the kit)
-edited (spelling and added an afterthought)