I would assume you can use any pick you own in the supplemental draft -- so any sort of trade would give you picks you can use. But there are several issues that make this unlikely. First, the draft is a closed process. Team submit bids (via email I understand) for the pick. The league reads the bids and awards the highest bidding team with the player. Teams don't know what each other are bidding. Second, trades would have to go through before the draft were to take place. You can't trade while you are "on the board" in this draft. Third, if you just traded for the first pick in the 3rd round, teams who want the player are going to know they need to use a higher pick. Fourth, you are trading into known slot in the draft with unknown picks. For example, if you wanted the Texans 3rd rounder -- the spot that Texans pick is in would be known but your own picks would have a yet to be determined location (given that the Texans weren't drafting anyone in the supp). This creates a situation where you are more likely to have to over pay. All in all if you wanted a guy and knew that another team ahead of you was going to use a 3rd on him, you'd probably be better off just using your 2nd to get him.