How do you determine that? Adding to a losing trade indicates to me that the main trend has been broken. That's why I don't add to losing trades.
At any given time there are many trends, what looks like a reversal on the 5 min chart could be just a pullback in the 60 min. As long as the reasons for buying or shorting are still valid, I don't see a problem with adding to a losing trade, it's a bargain, just make sure you keep risk in check at all times. Remember this site frowns upon adding to losers but never ever forget that most traders are losers, particularly those in a forum board. Averaging down is a powerful technique, but just like an atomic bomb, in the hands of a rookie, a complete self disaster. Don't let the myth keep you away from studying averaging down, just be smart about the maximum risk you are willing to allow and never ever do it against the main trend.