I personally will re-buy if allowed right at the start so i have double starting stack and then just play it like a freeze-out,i never add-on.
I wouldn't recommend this ^^ strategy (but hey.. if it works for ya.. great!). Rebuy tourneys have a strategy all their own. If you're not familiar with them but are considering playing them I'd suggest reading up on them.. &/or watching a training vid. of rebuy play.
Prior to gaining a better understanding of rebuy tourney strategy, I might've liked WONEGA's approach but after researching it a bit, I wouldn't advise it at all.
Rebuys are really about the tournament that starts after the rebuy period ends - - it's basically a deepstacked tourney favouring better players who are adept at deepstack MTT play. You will see some guys who appear to be complete maniacs during the rebuy period but for alot of them there is a means to their madness.... but then again there are a bunch of spew donks in there too.. lol.
'Generally speaking'.. it is suggested that when you consider a rebuy, think of it as being a higher buyin than what is stated (ie.,.. if it's a $10 rebuy, think of it like playing a ~$50 freezeout).
To learn more about your particular question, just open up any rebuy tourney lobby, then click on where it gives you details about the tournament (ie. 'blind levels', 'payout structure', etc.)... the details about the rebuy & addon will be in there.