It is feasible to buy store lambs once the store marts start that will be away by the back end, yes.
You would need a holding number and a flock number.
The main risk, I guess, is that one or more of the store lambs isn't ready by the back end - got lame and lost condition, or other illness ditto, or just is a poor do-er, or was of a type that needs longer to finish. So then you have to get off your grazing and have lambs that aren't ready to slaughter.