If computer A send command via serial port (RS232) you certainly can program a FMD88-10 (which is a T100MD888+ successor) to receive the serial commands and act on it.
What you need to know is the command format of the serial command from computer A and what each command mean so that you can program an FMD88-10 to take over the job of computer B.
There are some limitation which is that FMD88-10 supports only the following common baud rate with most PC:
9600, 19200,38400, 57600, 115200, 230400.
Also the PLC has a serial circular buffer of 256 bytes only. Presumably a single command from Computer A can fit into 256 bytes then you should be ok.