Put firewall hardware with ethernet ports between serial ports
Modbus is a serial communications protocol, a link layer standard. In your situation, it is being transported over RS-485, a physical layer standard. So, you might say that you have "Modbus over RS-485".

Ethernet/RS-485 converters take "Ethernet over copper", and turn it into "Ethernet over RS-485". RS-485 is the physical layer standard, Ethernet is the link layer standard. On the other end, the converter does the reverse.

What you are proposing uses the converters in reverse. That is, you would have a converter that expects "Ethernet over RS-485" and is instead receiving "Modbus over RS-485". 

Unless you have a very smart converter (and without more details about your hardware, I'm assuming it's not), I think the answer will be "no".