if the current water system is something like a pressurized system its fine, but if its run off a gravity feed it might not like it. or if its a power shower it might be fine.
i'm currently doing a bathroom at the moment, it had a bath and no shower, so i've plumbed in the pipes and just waiting to tile before i can fit the unit. but its a pressured system which makes it easy. as i can bolt what ever the hell i like on to it. as all i have to worry about is the 2 pipes coming out of the wall.
i'm setting it up as a twin shower, so a large rain type shower head and a 2nd shower head to be used in the bath etc.
i'm not sure you can get just a bolt on rain head; usually you have to buy the whole caboodle. with tap section etc.