It's not particularly painful. Grab TGA2TPC, load your TGA, set the export directory, set the compression to DXT5 (make sure your normal map has a solid white alpha), create an appropriate TXI or change the setting to text and paste it in the box. At a minimum you will need
isbumpmap 1
If you have modified the diffuse, then it will also need a TXI that includes
bumpmaptexture filenamehere
that points to the filename of the normal map.