This in a nutshell is what I did (can elaborate further if you need):
1) assume start with untextured model: open up material editor (all those lovely gray balls!), click on diffuse, point to bitmap (I use bmps) and you have texture (automatically on channel 1). Then just add to model. If doesn't quite fit (irregular object) that's when need to uvw map. Assume all light and textured from this point on.
2) Baking - open up render to texture. Scroll down and make diffuse map (tga), output to source. I now have rendered tga in place of my original bmp.
3) Lightmap - with my material open in editor and relevant bit of model of selected, open render to texture again. This time, under mapping coordinates, make sure it's 'use automatic unwrap', channel . Under Output, hit add and select either shadowmap or lightmap. Under Selected Common Element settings, make sure target map slot is self-illumation (channel 2). When you hit render, you should end up with a nice lightmap tga in your texture foldeer but which is automatically added to your original diffuse texture material ball but on channel 2 (self-illum).
Re: free slots in material editor, assume you mean no gray balls ? (lol) Anything you've done should be stored in library, so just turn any of your existing material balls you don't want to use back into standard gray. You can recall library maps back up anytime.