No reason why you couldn't splice in 2 more bulb holders & simply pop them in the outer sections.
If you have a cargo light 3rdBL, you'll need either red bulbs or red led bulbs in the outer sections.
If you get an all red 3rdBL, you'll need is 2 extra bulb holders (think I have some of them somewhere too).
About a foot of red & black wire. I'd solder the wires together rather than use those quick taps... especially if you're going to use incandescent bulbs. Shouldn't take too long to put that together.
I have 2 all red 3rdBL's completely LED modded, one has 112 leds, one has 63 leds.
The 112 3rdBL has an OEM wiring connector that simply plugs into the OEM harness.
The 63 led one has a single bulb base that plugs into your existing bulb holder/socket. No wiring required.
(end shameless plug)
I'm not a big fan of the led 3rdBL's I've seen on evilBay, that's why I started modding the OEM 3rdBL's with my own spin on things.
Check out my photobucket... I've got a quick vid of my newest 3rdBL where I have the 2 outer led panels connected to my turn signals/hazard lights.