hum... that's a too big work and considering your question you know few about how skins are made... *.abp file just tell where other files for a model are. If you edit abp file you'll see for exemple wehrmacht/wehrmacht for the wehrmacht grenadier. That means wehrmacht directory and wehrmacht.rgm file.
The rgm has to be edited with corsix studio, and this file point to real textures that are .rgt files usually in shared folder.
The only advise I can give is pick a model you know, and try to understand how it works and where files are... then you can start to modify stuffs.
But I have no easy way to change HSM... only hard work , sorry for that. I will take you a lot of time and sweat