Can you add another Modbus port on a TSX Momentum M1 Ethernet controller?

It depends on the CPU model. There is a limit of two Modbus ports on Momentum M1E controllers.
With the 171CCC980x0 M1E, the 1st port is Modbus TCP/IP and the 2nd port is Modbus RS485.

With the 171CCC960x0 M1E, the 1st port is Modbus TCP/IP. You can add a second Modbus port by utilizing a 172JNN21032 (Modbus Option Adapter). This port is software selectable Modbus RS232 or RS485.

Note: If you use the 172JNN21032 (Modbus Option Adapter) with the 171CCC980x0 M1E's, then this port is disabled.