Adding an extra 1ft to premium speaker cables

Intuitively, one would think it will defeat the purpose. If the cheap cable was to take away anything, there would be no way to put it back. However, since you are dealing with cheap cable, it would not cost much to give it a try and revert back to the original state if things don’t pan out. 
Keep in mind that the term cheap has different meanings depending on how deep your pockets are. Have you considered Ali Express cables? They seem to be over performing their price. 

Don't do it ...

I am curious why "there’s no way to change it", in regard to your needing one more foot.

Perhaps there is a solution that you haven't considered.

Only one way to be sure try it and find out.

If not sell them and use the proceeds towards wires long enough to do the job for you.

If you got them for free whatever they sell for is your bonus to use as you see fit.

If you try, just make sure you don't do anything that might hurt the resale value of these cables...what brand and model cables are they ?