Yes, that’s one of the very functions of the T3 controllers, they act as a transparent Modbus TCP to RTU (RS485 ) gateway. You just send a packet to the IP address of the T3 controller along with the Modbus ID and registers to poll and the T3 controller will pass the message to the subnet transparently. If there’s a match with the T3’s own internal Modbus ID then it will reply with its own register data.
In addition to the transparent mode described above you can also have a look at this forum post which shows how to program the subnet devices using a T3 controller as a gateway. You write a program in the T3 controller to poll subnet devices and massage the data if you like, it operates like a ‘programmable gateway’. You can also use your own gateway if you have one in mind.