Adding a zerk is a very simple process, here are the basics:

• Remove the axle shaft, you may need to take apart the brakes to do this.
• Grind off the rust and crud off a small section on top of the axle tube between the bearing race and inner axle seal.
• Make measurements to determine where a zerk can be installed and use a center punch to mark the spot.
• Drill a 7/32" hole for 1/4-28 zerk fitting.
• Tap the hole with a 1/4-28 tap.
• Screw in the zerk.
• Replace axle shaft and brakes.

After everything has been installed, you can use a regular grease gun to pack the bearings. You may want to pull the drum to keep an eye on the outer seal. If you overpack the bearings, grease will ooze out and ruin your brake shoes. Greasy shoes have very poor stopping power.