I didn't use a kit, I simply replaced the ballasts (they were old magnetic ones, of which one didn't work and were for 18w bulbs) with an ADVANCE electronic ballast that fires 4-32w T8 bulbs (available at homedepot pro shop.. maybe regular one), a little wiring, with a socket holder I had around, and bammo done. However if you simply want to add a third bulb, you could simply use the 2 bulb ballast they sell (or 4 and overdrive them by adding 2 leads to each bulb!! although the bulbs will get noticeably hotter). I had to move the sockets for the bulbs against the outside wall as well since the 18w bulbs were shorter than 24w bulbs, but since you're already using 24w bulbs shouldn't be a problem.
Main compelling reason for only doing 3 bulbs, for now is because the acrylic "splash guard" could fit 3 in it, 1 space was reserved for the old ballast, but since I wired the new one outside the cube I tossed a bulb in there, and there did seem to be quite a bit of condensation from evaporation on the splash guard so thought it best to keep all that electronic stuff behind plastic. That and I literally wired the whole hood together the night before the swap, so kind of did it as a rush job without wanting to fabricate or cut any acrylic for a new splash guard.
But the reality of it, a cheap home depot brand ballast for T8 bulbs seems to fire the 24w bulbs at roughly the correct wattage (I checked with a kill-a-watt meter) and if anything runs them a little cooler extending the life of the bulbs. And get yourself some PC bulb sockets and you can modify anything, just need to do a bit of wiring which isn't much of a problem.
). You're right about temperature though, that might be the deciding factor (depends how warm the room is normally as well!), unfortunately I don't have any actual experience with nanos & MH lighting (not much with nanos in general actually 
