I don't think SFM has the capability or any built in tool to do that.
From my understanding SFM is running a heavily modified version of the Source Engine, so it's essentially a "game engine" with what it can handle.
SFM wasn't really designed to be used with high poly meshes. It was designed with the fairly minimal tf2 models in mind.
I believe 10k polygons is the limit, at which models will not compile into source. Most recent compilers for SFM can work around this, however, by splitting your model into 10k poly segments, and clamping them back together.
The only real solution is to subdivide your mesh in an external program, and even then, most compilers will reject models over 300k polys.
Even in cases like that, your flexes may break when compiled in SFM, as there is a 10k poly limit on what can be affected by flexes.
tl;dr, it's very difficult, and sometimes flat out impossible>>535730
Reskin as in re-weight, or reskin as in textures?
Textures are located in
SourceFilmmaker\game\usermod (or wherever you played the models)\materials\models(?)
(sometimes modelers are lazy assholes and just dump them in the base materials folder)
You need a program called VTFEdit to convert textures to and from VTF.
Re-weighting models requires decompiling them and fucking around in an external program, and then re-compiling them back into Source.