Since Modbus TCP/IP is simply Modbus protocol with a TCP wrapper, it is very simple for existing Modbus devices to communicate over Modbus TCP/IP. A gateway device is required to convert from the current physical layer (RS232, RS485 or others) to Ethernet and to convert Modbus protocol to Modbus TCP/IP.
Such a gateway device could be implemented using a PC. Commercial products to do this are available from several different manufactures. The Modbus device database can help you identify gateways and other Modbus devices.