No it's not that easy, if you are using a high powered horn (Nautilus mini maybe?). The original circuit is fused at 15A, so you shouldn't aim for more than about 10A or so. The Nautilus is 18A.
So you will need to add the relay supplied by the aftermarket horn maker. This will require a small amount of electrical work, and should be fitted after the existing relay so the alarm functions still work. If this fills you with dread, probably best get an autoelectrician to wire it up.