How many people usually add-on? If they get 10K chips before the 1000-2000 level, then that's 5 big blinds. I'm not sure I'd add-on in that situation.
If you get the amount of add-ons you want then don't change anything and you can stop reading now.
If not, then I suggest increasing the add-on stack. I prefer having a rebuy stack of 1.5 starting stacks.
Even if you were to change to 20K or 25K it would still only be 10 or 12.5 BBs, so perhaps end rebuys one or two rounds earlier? A drawback of shorter rebuy period is if course that a few less players will rebuy and also the poker night will be shorter for a few. A drawback for larger rebuy stacks is that if you want to meet your deadline you'll have to speed up the structure, since more people will be adding on and getting more chips for it.
My 5 cents