This is not directly supported by 3CX. However, 3CX permits you to setup a Generic gateway. You might want to try that, but if there are problems once you have licensed 3CX, you might find support somewhat limited.
Outgoing calls should be straightforward enough. With incoming calls you need to tell the gateway where to send them, usually the internal trunk number on 3CX. Check out some of the supported gateways to get an idea of how to best configure it.