Actually yes you can. Center channel speakers were designed to be low profile and have a large listening angle. If you want to use a bookshelf speaker, you should not turn it on its side(assuming its a standard single woofer-tweeter design) as that would cause breakup nodes along the horizontal listening axis(which is a more important axis because typically not everyone is sitting perfectly in line with the speaker.

A bookshelf speaker placed in its normal vertical position will typically not have the large off-axis response that a center channel speaker would. However, it will have a more even frequency response with less areas of cancellation between the multiple woofers.

Basically, go ahead and do it, but make sure you leave the bookshelf speaker upright and try to sit as close to inline with the speaker as possible as this will give you the best sound possible.