I've disassembled a couple of my cartomizers, and tried a couple of different ways of refilling/topping-off. What I've found to work best so far is:
1. Re-insert the cartomizer storage plug in the battery end of the cartomizer, and then cover with a cartomizer storage cap.
2. Using a tiny screwdriver or something comparable, gently pry out the white cap - while not sticking your pry tool in the center hole any more than is necessary to get the job done. (The white cap is actually a disc glued to a ring; so you want to be careful in removing it. If you pry too hard, and pull off only the disc, then you won't be able to put the white cap back on without re-creating the cap by re-gluing the disc to the ring.)
3. Using the same pry tool, gently pry up, and out, the little silicone ring - while avoiding any contact with the center wire that holds the heating coil.
4. Using a syringe with a needle, inject about a half a ml or more into the the outer ring of white material - sticking the needle inbetween the white material and the inner wall of the cartomizer body. I put about a quarter ml in on one side, and then shift to the opposite side to finish - which is once a sufficient degree of saturation appears to have been achieved. (I haven't actually tried it yet, but dripping a couple of small drops down the center hole as well may not be a bad idea.)
5. Re-insert the silicone ring into the cartomizer body, and gently push down in an even manner until it sits just above the white material.
6. Re-install the white cap, and the second cartomizer storage cap, and what you have is a good as new cartomizer for future use.