While per the rules it is technically allowed, it is not in the spirit of a game jam, and will get rid of all the fun of making a game during the specified time period.
Also note that there is an extremely big chance that your already made game does not fit the theme at all.
Do yourself a favor, if you want to participate, try and make a game specifically for the jam, you don't have to finish it, you don't have to even submit it. The learning you'll be doing during it is what counts much more than anything.