I don't see why this would be a good idea, but I assume you have space problems that precludes you from having the front three speakers, while you have space for the back surrounds?
In that case, you could look into getting something like the B&W Zeppelin and run the FL, C, and FR channels into it with your pre/pro set on "phantom center". The Zeppelin doesn't have RCA inputs, rather it only has a stereo 3.5mm mini jack for auxiliary, so you would need to get a RCA-to-mini jack cable/adapter to run the line-out for the FL and FR channels from your pre/pro.
You could also get a soundbar, but generally soundbars use digital processing (i.e. Dolby Pro-Logic II) to take stereo signals (which it would be if you were running "phantom center" from your pre/pro and not include the surround channels) and "convert" them to "surround sound". So it would be odd if you were getting a "surround" soundstage from your front three channels in addition to the actual surrounds channels. I've never heard such a set-up so I don't know what it would actually sound like, but generally the "surround" effects of soundbars don't work well, especially if you don't have an ideal room for it, so it may not matter.