No offense, but that’s a really bad idea.
You need a discrete centre channel signal to run a centre speaker.
If you only have a stereo signal, you achieve the impression of a centre image within the soundstage by placing your left and right speakers correctly. When that happens, your stereo image will give the impression that the vocals are coming from between your left and right speakers.
Feeding a combination of the left and right signals into a centre speaker would sound dreadful, if it didn’t damage your amps up in the process.
By all means consider using a subwoofer in your setup, but centre speakers are the sole preserve of a multichannel system with a 5.1 or above signal, feeding a multichannel amp (or pre-amp/processor into power amp(s)), where there is a separate signal for the centre channel information within the mix.
So typically, a movie soundtrack that would carry much of the dialogue of the movie.
If the signal is stereo, you need to play it in stereo, or use a multichannel AV receiver that has the capability to upmix a stereo signal into a multichannel one. Generally, such upmixers don’t work especially well anyway so I couldn’t really recommend that.
If you really want to introduce a centre speaker, then you need a multichannel AV receiver with pre-outs or pre-amp/processor with its own discrete centre channel output, not a stereo amplifier. Then you would run that single RCA cable into the centre channel input on your power amp.