How about adding this by Jim825?
When I've created All Star teams for the single season mods I released (TC1951, TC1956, TC1978), I've done it as a multi-step process. Here's how I did it:
1. If you already have a .mbe file, make a copy of the .mbe file. If you don't have a .mbe file, open up MVPEdit, import your rosters, and save the .mbe file. Exit MVPEdit and make a copy of the .mbe file.
2. Open up MVPEdit and open up your copied .mbe file. Create your All Star teams by moving players from their current teams to the All Star teams. Don't worry that they are moving from their original teams because you will be discarding the .mbe file copy later.
3. Once you have created your All Star teams, export the two teams using the File->Export->Team menu option. Do this twice (once for each team) and save each team as a .met file, giving them names like AL_AllStars.met and NL_AllStars.met.
4. After you have saved your All Star teams, open your original .mbe file in MVPEdit..
5. Import your All Star teams into the .mbe file using the File->Import->Team menu option.
6. After your All Star teams have been imported, export your rosters to your data database folder using the Export menu option.
You now have proper All Star teams and the players are also still with their original teams.
This has worked for the single season mods, where I play exhibition games or run dynasties (with all trades and injuries turned off). I can't guarantee what would happen if injuries occur or trades are allowed, but for me, this works fine.