In my opinion, none of the other answers addresses the obvious misconception in the question:

There is no use in converting a JPG (comparatively low quality) to a RAW file (high quality), because you do not gain anything.

The reason why people shoot in RAW is that, as others have stated, RAW captures all the sensor data and saves it in a file. JPGs have less information than RAW and that is why people shoot RAW in the first place. You gain nothing by converting a JPG to a RAW, because you still have the same information as before.

(Edit: JPGs have less info compared to RAW when shot on the same camera. A high quality (low compression) JPG from a camera with a good sensor can have much better "quality" than a RAW file from an old junk camera. Thanks to @mattdm for the comment.)

But don't be discouraged: Even though RAW files are better to work with, you can still use JPGs in photo editing and improve your old photos. It will be significantly harder to make large jumps without creating artifacts, though, so maybe less is more here when you are editing.

TL;DR You can do something with your old JPGs, but you don't have to convert them to RAW for that.