Yes, as long as the PLC program is not actively sending any modbus command out of COM1, the communication port is immediately available as a slave port. i.e. the same comm port can work both as a master and a slave.
You will have to take care of the "arbitration" issue, meaning that you have to ensure that the PLC will not talk on the COM1 when it is supposed to listening to incoming command. Otherwise a collision will occur and both incoming and outgoing command will be lost.