You cannot accept unknown SIM cards due to mutual authentication -- the provider must know the shared secret that is burned into the SIM.

You can achieve network separation APNs, so there is no need for such a hack.

The question I have in mind is: Does your office advertise it's own PLMN with WiFi-like equipment, or do you outsource your office wireless network to an established operator which handles authentication and provides a virtual, separate network?