Dual crown fork or not?
Putting a longer travel fork on a bike not designed for it will alter the geometry of the bike, and usually the handling.  Depending on the amount of change, it might be barely noticeable, or it could also appreciably degrade the ability of the bike to be ridden.  You could put a 200mm travel fork on a road frame, but the resulting geometry would render the bike unfit for anything serious.

The change you are talking about will likely result in a rideable but slightly awkward ride.  It's unlikely you'll snap your frame (highly unlikely).  However, it's far more likely that if you use the front fork (a 200mm dual crown monster) to it's full potential, you blow out/prematurely wear the rear shock which is not designed for that kind of abuse.  Considering what a fork like that (or a replacement rear shock) costs, you'd probably be better off selling that bike as is and trying to by a bike more suited to the type of riding you are planning.