When you have the main quest to convince the Jarl of Whiterun to trap a Dragon in Dragonsreach, I've read that if I haven't sided with either the Stormcloaks or the Imperial Legion that I can call for a temporary truce between the two factions until the Dragon issues are dealt with.

To my question: Is it possible to join one of them AFTER the the truce has been called and after the main quest line has been finished?

Thanks in advance

Yes, its a truce until the Dragonborn deals with the Dragons, then it resumes.