I wouldn't do it! Let what you have finish, rack to sec or bottle. I wouldn't add a thing after the yeast started except hops, fruit, etc.
Be happy with 4 gal or good beer instead of risking having 5 gal of mediocre beer.
"I brew with a water cooler and some part from the toilet." - JohnnyO
"I do gravity feed the last gallon or two through my Therminator, but I expect you could suck start a Volkswagen before you could suck start one of these. - GilaMinumBeer
"..... Bull was right." - TXCurtis