If you mean for your SENDS to others (which is when you need an incomming port), then yes, just go alt-o/connect/options/advanced and set the first and last one in there.
If your going to send from diffent pc's behind the router (and im assuming these pc's have static IP's not dynamic dhcp ones) Then i would suggest each pc be assigned a port to be forwarded to it, and the mirc on that machine set to use that port.
Alturnatively, you can use Port trriggering options if your router has it, I have used port 6667 being the normal IRC serverport, when the router detects a PC using this it well on the fly open a port or range of ports for inbound traffic to that PC, this way all the mircs on all the pc's can use the same dcc port range, this has a downfall in that if u run two mirc on 2 pc's i cant predict what your router might do. This is a far more complexe operation to setup as well.
