Do you mean you want to put the 12" woofers in your floorstanding speaker cabinet? If this is the case you need to find a woofer with specs close to the one that came out of it. Not only does the woofer need to fit the cabinet volume but it also needs to fit the criteria the crossover was designed for. It would also need to match the original woofers sensitivity, too low and you might not hear it over the loudness of the mids/highs, too high and you may not have much mids/highs.

If you are looking to build a sub I personally would look for one that would sound good in a much smaller cabinet.