A lock with the key over ride is a must when there in only one entrance/exit. Without the combination there is no way to know if the lock is operating properly. Combination it is a very involved process to reset the lock to accept a new combination. Back be removed, the lock disassembled, the combination chamber removed and the tumblers reset. FYI, if they don't have the keys a locksmith can not just make a key.
The over ride lock core will have to. The item "Kaba Simplex LL1041 Pushbutton Lock Over Ride Core Installed withKeys Also Passage" is in sale since Monday, October 09, 2017.This item is in the category "Business & Industrial\Construction\Building Materials & Supplies\Doors & Fixtures\Locks & Locksets". The seller is "bake98" and is located in Oxford, Pennsylvania. This item can be shipped to United States.