Adding a center channel

Currently have 2 channel system, with 2 amps, stereo preamp, tuner, etc. Would like to add a center channel, is there any way to accomplish this without buying a new preamp? I've heard there are processors, but is there one that would accomplish what I'm looking to do? Any suggestions or recommendations??
I assume you have two stereo amps (and not monos) and are looking to add the center channel for home theater. You'll need a a component to handle the preamp function between the source component (DVD player, VCR) and the center channel's amp. I recommend buying a preamp/processor. You'll also need the processor for at least DTS and DD decoding (although most DVD players have built-in decoding for at least the latter). And you'll need a processor for VHS viewing in Dolby Pro-Logic.
Sounds like you will be needing a H/T pre/processor if your system is evolving for home theatre applications. Buy one with a pass through so you can still utilize your stereo pre for stereo music. Don't go cheap on the center speaker either. It took me 3 upgrades to learn that lesson. I think you may also need another amp but there you may have options. What is your main objective down the road? Maybe I can help. I guarantee I have already made every mistake you are about to make. TWICE
This is not your usual audio nut response but here goes. Buy a inexpensive DD processor (mine is an Onkyo) that has two inputs for DVd and laser/vhs. For laser discs you will need a RF demodulator. Buy a CC speaker that is similar to your own. Mine are matrix B&W's, could not afford their CC so I listened and bought an Energy RVS-1, which had a similar character to my ears. You need 5 channels of amplification, so two stereo amps won't cut it. Run your DD processor speaker multi-output to the inputs on your amps. Run your two channel speakers from your preamp output to your amps with separate RCA connectors. Simply pull out the two front L and R from the DD processor that goes into the amps and insert the preamp outs for two channel listening and vice versa for movies. It works for me and both systems although close are separate. By the way once you start down the DD path you are going to want a subwoofer. Trust me, watching Terminator 2 without a sub is like kissing your girlfrien thru a screen.