Using the pump to augment city water won't hurt the pump, Darrell. We've done it many times when more pressure was needed for some endeavor, or the park's pressure just wasn't up to snuff. Our owners manual even says straight out that it's okay in the section about how to use city water, and we used to do it all the time on our old '84 Pace Arrow. It's best, of course, to see to it you do have water in the tank for the pump to use during the boost.
Joel