I've never bought the boxed kits, but from reading other people's posts, it seems like most kits include an info sheet that will give you expected OG - I'm not sure about IBU.
Two options, you could simply weigh the bag of extract they include and add 43% more extract or 3 lbs. (since 5 is 43% more than 3.5). Or...
Let's say the kit is supposed to have an OG of 1.090 at 3.5 gal
1.090 * 3.5 g = 315 BU
1.090 * 5 g = 450 BU <-- this is what you're shooting for
450 - 315 = 135 GU diff <-- so, this is what you have to make up with the addition
135 BU / 45 (ppg of DME) = 3 lbs. of DME <-- amount of extract needed to match OG with 5 gal batch
Like I said, you will need to adjust the hopping also. I'm not sure what volume you are boiling, but you could do roughly the same math and just bump up the amount of hops for each addition by a constant math. Unfortunately, the appropriate amount could be anywhere from 35% to 50%, depending on several factors (boil vol, wort gravity, etc.)