Depending on your location, you may not want to add a thing. The original standard configuration would have a drain hose bib on the low point of the plumbing, just after it exits the house, and upstream of the PVB. Nowadays, this faucet could be frowned upon, as a possible cross-connection, where the codes are making the assumption that some idiot will hook up a hose and connect it to something nasty. One way around this is to use a simple PVC plug in place of a boiler drain faucet. Remove plug, connect air, winterize, replace plug.
It is more than likely that local service companies already have any adapters they need to connect their air to a PVB testcock, which means the indoor drain doesn't have to be utilized for winterizing.