Most 50w oil comes in a container with a tapered nozzle like a tube of caulk. Just cut the tip off for a small hole.
Tips:
Do it in warm weather. The hubs will drain much faster. It will take all day if it's below 50 degrees out.
Put the container of oil in the microwave for a few seconds to warm it up. It will fill up much faster that way - trust me!
Fill the hub, let it sit 10 15 mins, then do it again. Repeat until it's at the desired level after sitting for 10-15 mins. Just don't fill it up completely full. I know the hubs pop out and expand when the hubs really heat up, but that's still not enough expansion for a totally full hub. The pop-out mechanism is really just for extreme conditions, and not something you should see happen on every trip. If your hub is too full, you risk popping the back seals.
And yes, dip the threads in high-heat sealant before reinstalling. Otherwise it will leak everywhere.