Early in the life of USB keyboards were sold that were really capable operating with both USB and PS/2 interfaces, but came with an PS/2 connectors. If you wanted to use them as USB keyboard you needed a small adapter, containing nothing but wiring but which converted the PS/2 connector to USB.
That means that with such a simply adapter your "PS/2" device MUST also contain USB logic, or it simply won't work.
If you have an "active" adapter that contains the logic for an USB to PS/2 converter, then yes, you should be able to use it with a real PS/2 (only) keyboard.
These days read USB keyboards and mice are so cheap that almost nobody sells such adapters anymore.