Franky, I'm interested in what you're saying but I don't really understand - I'm relatively new to Photoshop, and having a French copy is an issue when most tutorials are in English.
Could you explain what are the texture baking method and the diffuse? I usually hand paint my bumpmaps on a desaturated copy of the main texture because I don't like the results of the NVIDIA normal map filter, but it's rather a pain.

Szynszyl,
to bumpmap-enable a custom mesh you need to make it go through Skankyboy's smd exporter in SimPE:
- open the mesh, select the gmdc and click "export" in the plugin view, then choose Skankyboy's smd exporter,
- import it right back, checking the right number of assignments in the pop up window and "create bumpmap", then select "replace" in the next window.
You need to do this for each group separately if there are more than one, same for the morphs is the mesh has any (select the morph number in the first pop-up window on import).