yep,
know your way around the Editor?
click on competitions, find european champions cup, click on registered teams, find Celtic and replace them with a different team (a team not currently in a european competition, wolves for example)
then find EURO cup, registered teams and then put celtic in place of one of the clubs listed..
Note: important that you just 'replace' teams, if you use the 'add' or the 'remove' buttons you may end up with too many or too few clubs for the competitions to work and your game will crash