Simply add a PCI card that has some serial ports. There are many manufacturers that make such cards. I don't know your level of programming ability, but I/O code is not that difficult, although not often utilized by the typical programmer... so many people don't know how to handle I/O. There are some good texts out there that address I/O specifically if you need to bone up.

e.g. "JAVA I/O" by Elliotte Rusty Harold if you are JAVA equipped.