Not usually, you can often add a faster processor but to have a dual processor 'rig' requires a special motherboard. (what's your current PC specs - CPU, Motherboard Model, RAM etc?)
a few years ago you used to be able to add a PCI card with a processor on it, this processor would then work in parallel with your existing processor, but i don't know if any are available today, will have a look and get back to you
You'll need a PCI-X slot for the above option. This is a PCI eXtended, not to be confused with the newer PCIe ---PCI Express interface. It may be possible to upgrade what you already have, but that depends on what motherboard you have.
yes B's right, i've found another PCI-X card http://www.commell.com.tw/News/News/News_20050711_HS-770.htm i think the only other option is a multi processor MB (up to 4)