hi mate
i am trying to do the same
i will tell you what i know which isn't much
you have to first get fc united into the game.
i did it using the editor but it took me ages it kept freezing and i have lost count of the times i have had to reinstall the game.
anyway i finally got fc united created and in the game but not in a league.
then using notepad
create a text file name it "FC_UTD.edt", write the line ...
"SWAP_TEAM" "X" "Y"
where x is FC utd's ID, and Y is the ID of the team you would like to replace. Make sure this is in the right folder (C

Program Files\Sports Interactive\Football Manager 200*\data\db)
i have done this started a new game but i am getting the error message
cannot swap teams as one maybe extinct or invalid.
i think i am on the right lines but i am still trying to get some help from someone to solve this error message.
hope this helps in some way if i find out any more i will share it with you
ralphie