A deal for the Stockport County midfielder looked to have hit the buffers earlier in the week but a fee has now been agreed with the relegated Hatters and talks will begin with the player.
Turnbull came through the ranks at Edgeley Park to become Stockport skipper and was one of the few bright spots of their season in 2010/11.
His capture would represent another piece in the jigsaw for Cobblers boss Gary Johnson after the deal to land freed Newcastle defender Ben Tozer.