Center channel speaker and stereo old receiver?

No. Do not jump the two speaker outputs. It could result in a room full of smoke. Now, there is another procedure that is a bit less risky although I have to tell you that there is some risk. Connect the + for the left channel to one of the speaker wires for the center channel and the - for the right channel to the other center channel speaker wire. It doesn't matter which speaker wire. The + speaker wire into the left + and the - speaker wire into the right + or the reverse will work equally well. This will give you what is in effect left minus right. It's not a true center channel surround sound but it works. Ive done it and none other than Brian Eno recommended this procedure for his "Ambient" series of recordings well before surround sound came out. As I said, it's not without risk but it is safer if the receiver is rated to power 4 Ohm speakers and you are using 8 Ohm speakers. But this procedure is intended to be used with three speakers. Left, right and the "Ambient" or center channel. I can't say exactly how loudly a single speaker will play this way or if it will be a safe thing to do. If nothing else, you should get yourself a cheap pair of 8 Ohm speakers to use as left and right.