Can I setup insulation without tearing down walls?
This is done all the time by insulation contractors. The technique is to drill a series of holes at the top of the wall. They next use a commercial 'blower' that forces insulation into the walls bay. It is a very non-invasive form of insulating because (as you asked) it doesn't necessitate the demolition of existing walls. It is also well with-in a competent DIY-ers capabilities and machines can be rented at most home centers.