As I read your question, you want to know if you can use your pre-order bonus MW3 token to unlock an additional custom class; not be forced to use it to prestige to the next level. If that's what you're asking, then the answer is yes. Simply go into the prestige shop and choose the option to unlock an additional class.
You'll never be forced to prestige up; in fact, Activision wouldn't allow you to use the bonus token to prestige up another level even if you wanted to. Remember (pre-order bonuses aside), tokens are your reward(s) for playing through 80 levels and prestiging up. They unlock items for immediate use in your new prestige; since you re-start each one at level 1. You can't, for example though, complete 4th prestige, then turn right around and use the token you just earned to skip 5th prestige and instantly jump to 6th.
Hope this helps.
Keep your powder dry,
Wolf
Message was edited by: Wolf_Odinson
Message was edited by: Wolf_Odinson