That works just fine as long as the waterfall isn't too large and the plumbing is setup appropriately. You don't want to run the waterfall water through the filter, so the waterfall should branch off before the filter. Also, if you originally had two pumps you want to combine their suction lines so both feed the one pump, which will require a little extra valving so you can prime the two lines separately. If the waterfall currently has a pump larger then 2 HP, it will probably be too much to combine it with the circulation pump.