Absolutely not an issue. I use a Comtrol RTS-1 which is an Ethernet to serial adapter. This is a single port version, but they also make others in various port sizes. Simple to setup and has a port viewer function that allows you to see the communications between 3CX and the PMS in real-time. Been using them for over 5 years and never had one fail.
I do not have experience with the FOSSE system and a Google search did not reveal any details of how the PMS to PBX function works. I can only caution you to make certain that the protocol matches that which 3CX supports.