Yes, you can use a PLC to talk to Mach via modbus and 232, or 485 (with adapter)
You can have the PLC do all the ATC changing work, with mach sending start, and position, and plc sending done.
OR, you can do all the programming as an M6 in mach, and have the PLC just be dumb I/O.
I use ADC 06's PLC's for my complex ATC's, and I use Arturos Modbus boards for less complex systems.
The good thing with the PLC, is you can use the built in counter to count your Encoder counts from your ATC, and then send start/stop to a simple electric motor for a Cheap way. More expensive is a Servo driven system.
You can have mach control that directly or use a HS module.......
there are many options, the question would be make a list of what you want it to do, and how much you want to spend.
pass that info on to us, we can help you with some desicions and codieng help.
scott