No, you cant use a sound bar as a center speaker.
A sound bard is 2.0 audio, a center speaker is 1.0. RCA is left and right plugs, the center of an individual plug being +, and the outside rim being the -; thus rca has 2 + and 2 - while a speaker is 1 + and 1 -.
Car guys use what is called a LOC, Line Output Converter, to convert a PAIR of speakers to an RCA connection. The end result of trying to use this without a second set of speakers would sound just awful.
You also cant just wire the left and right to provide center speaker because true 5.1 sound has the center speaker doing most of the dialog and the left and right doing foreground sounds so they are very different sounds.
To save yourself time and money in the future, I would suggest doing your research first and your purchase second. Fire, Ready, Aim almost never works out well in the end.