Well technically you are talking about making a new model, just one that has everything the old model had and then some.
I’m not going to go too in depth but this is a rough guideline. I’d suggest reading up at the valve developer community/google if you don’t know these terms.
You’ll need to
-decompile the existing model
-load up the existing components into a modeling program (blender/max/maya/milkshape/etc)
-fit and rig the new components onto the existing model (alternatively, if the new components are on the same skeleton, just fit the mesh)
-split the model into groups and export each group separately, each with the full skeleton but as multiple .smd files (such as a ‘head.smd’, ‘shirtA.smd’, ‘shirtB.smd’, ‘pants.smd’, etc)
-you’ll need to modify the decompiled .qc toa account for the new smds and set up your bodygroups, the vdc has a good article on this.
then you should have a model with bodygroups. If that doesn’t sound too bad, then get reading, if it does, you might want to hire/beg someone who does this already.