Without using two ds I already use the gts but I want to add to the pokedex
Well in theory you could do it over the GTS, just put your pokemon up for trade, then switch games and trade for it.
But you'd have to be fairly quick in case someone else saw it and did a trade while you were switching games. Be as specific as you can when asking for the pokemon to trade (e.g. under level 10 or whatever).