Yes you can install in existing concrete. There are a couple ways to do this. 1) Use epoxy concrete anchors - hammer drill into your concrete and mount to the manufacturer's specs. You will want to make sure you are far enough away from the edge of concrete and that the existing concrete is thick enough. If option 1 does not sound like it will work you can cut a hole is your existing concrete and then dig the required 3 feet and fill up with concrete flush with the existing concrete slab. This is a very clean way of installing in existing concrete and there are no restrictions.
Depending on the situation you can install in existing concrete. You can use a concrete saw or jackhammer to make a hole in the concrete and then follow the normal installation procedure to dig down 3ft and pour concrete all the way up flush with the playing surface. Most of the installers on our installation directory (located on our website) will offer this service.