In an effort to make use of my PS3's digital audio, I purchased a new Sony 5.1 receiver that had the digital input. No problems there, and I was able to connect my front, rear, and center speakers without issue.

The problem is with the subwoofer. The previous HT setup was Panasonic, and the subwoofer used a normal +/- speaker cable. However, this Sony receiver doesn't have a cable connection for the subwoofer. Instead it looks like an audio plug (not an RCA connecter - more like the kind you use with headphones).

My question is - is it possible to put a plug on the end of the speaker wire so that I can use this subwoofer?

That's a pre-out for a powered subwoofer. A powered subwoofer will be louder than one powered by your receiver. Does your subwoofer plug in or is it stand-alone? (See reference picture, below) I've never seen a powered subwoofer that didn't have a line input.

However, if your subwoofer does not have a line in, they generally have L and R speaker inputs and outputs and have a built-in crossover. (See pic) You should be able to run your L and R speaker wires to the subwoofer and then onto the speakers.

If worse comes to worse, you can run two sets of wires from one of your main speakers, with one going to the subwoofer. This method is NOT recommended and may end up damaging your receiver.

I'd suggest a new subwoofer that includes a line in.

Reference pictures:

My current subwoofer (Panasonic) is stand alone and only used the +/- wire from the receiver. I guess I can break down and buy a powered one. Bummer.

You'll only be bummed until you plug it in. Then you will realize what a key piece of equipment a subwoofer is in a HT. Since you have to buy one, don't cheap out or don't bother getting one. Search this forum for a few good subwoofer threads for some recommendations.