yes, select the "all files" type under "save as" and add a TGA extension.
If your modifying a texture....well first off all i suggest you get Photoshop or Paintshop pro....both can be found about on the net, but you have a high chance of getting a virus with it. If you can afford to buy it, then do so. IMO Photoshop is the better of the two, its also the industry standard.
Then just open up your TGA and.....well, you will just have to play and get used to the tool.
Try drawing over the top of the old textures. I dont much modify existing textures (i make new ones) so i cant really advise any good modification techniques.