Can a FELV circuit be connected to a SELV circuit?
The first solution I would try is routing an isolation slot under your relays to boost the board creepage distance.  This by itself may be enough to bring your motor controller to a SELV level of safety; however, if the problem is the relays themselves (are they too small, or have poor isolation ratings?) then using an isolator on the RS-485/ModBus connection (in between the UART and the RS-485 transceiver for traditional approaches, or integrated into the RS-485 transceiver if you can't wedge it in any other way).

If those don't apply in your case, then I would say no -- most communications-type cables are not rated for mains voltages.  (They are a type CL or CM in UL parlance, vs. say NM mains wiring.)  A control network like this would likely be considered a Class 2 circuit (at least over here in the US, your electrical code probably has a passage equivalent in function to NEC Article 725, just written differently), and those have strict energy limits, similar to the concept of SELV where you are, in order to maintain their Class-2-ness (which is what allows them to be wired the way they are).  Lose that, and you downgrade to Class 1, which allows for higher energy, but requires mains-type wire and must be isolated from Class 2 and Class 3 circuits.  This is analogous to FELV in your case -- the circuit may run at low voltages, but you can't guarantee that the voltage will always be limited in case something goes wrong.