If you connect a speaker to line level converter
https
/www.crutchfield.com/p_142LOC80/Scosche-LOC80.html
to the center channel speaker terminals of the receiver (both channels of the converter to the same two terminals) and connect that to the analog input of the soundbar you could use it as a center channel. It may not work well but if you already have the soundbar the converter is cheap enough.
If you don't already own it buy the correct matching Polk center instead. It will sound much better.
The Polk towers probably have more and better bass than the soundbar sub so set the front L/R to large and the center to small.