No. In most cases, the rain cap will not physically attach. Even if it does seemingly connect, rainwater or condensate may leak back into the vent system and cause corrosion. An exception to this is the Noritz Universal Rain Cap which will work with the EZTR venting system.
Condensate
Liquid water that forms when the water vapor, developed as a product of combustion, contained in exhaust gases cools enough to change into a liquid.
Corrosion
the disintegration of a manufactured component caused by chemical reactions with its surroundings (e.g. rusting)