Signing Lamar Jackson

I’m hoping somebody that knows the rules better than me can clarify an idea I had.

If the Colts wanted to sign Lamar they would send their next two first round picks to Baltimore. My question is, could they trade the #4 pick back into the late 20’s and pick up more picks, then sign Lamar to a contract? So instead of sending #4 and next years pick they would be sending say #29 and next years pick.

This doesn’t seem like it should be allowed, but I don’t know if there is a rule against it.