Cannot comment on LeoCAD as I've never used that, but LDCad is far superior over MLCad and it works well on Linux natively. LDCad uses same part library format as MLCad - LDraw. You can find LDraw file documentation here.
It is not clear from your question if you wish to create your own parts or just add the ones not currently available in the official LDraw package. If you are interested in the latter, there is a guide. It can be easily adapted to Linux.
If your intention is to create your own parts, LDraw contains nice documentation explaining the file format. The process of adding a newly created part into the library is the same as explained in the section above. Just make sure to put your file for the new part into the
.../Unofficial/parts directory under your LDraw package installation directory and make sure to run
make-list to include the newly added files in the list of available bricks to build with.