Adding can lights to an existing ceiling can be a relatively easy process, depending on the type of ceiling you have. If you have a drywall ceiling, for example, then adding can lights will simply involve cutting holes in the ceiling and installing the fixtures. If you have a plaster ceiling, however, the process may be more difficult as you'll need to locate the studs in order to properly secure the fixtures. In either case, it's always best to consult with an electrician before undertaking any electrical work.