You must find center speaker which timbre matched to your front speakers or you'll loose any hope to get home theater effect, since effects transition won't be seamless from center to fronts.
Pretty much only sane way to do above is to buy speakers from same maker AND series.
If design (WAF) is your major concern you might want to consider selling your current fronts and getting all new Left/Center/Right speakers setup which match your design requirements and sound preferences