If I recall correctly, you can enter a "USER" satellite at the end of the sat table, but you can't get the Angular location entered. If that's the case, you'd have to use Diseqc 1.2 mode for locating the bird. Might also just rename Galaxy 14 and use that slot for AMC21 , if you're not a C-bander.
you can use the editor. Its posted here somewhere (I forget where) but it screws up the USALS. If you just started it might be the best to do that (use the editor) or just rename a C-band satellite as noted above