There are several examples of how build a collar on a keezer, which I and others have commented on in the past. Visualize your kegerator as a vertical keezer. Use 2" x 8" s to build a collar or use 2" x 10"s, whatever you prefer. Remove the front door. Attach the collar to the Kegerator using deck hardward brackets you can pick up at Home Depot. Add 1/2" thick solid core foam insulation to the inside of the collar to help the cooling ability of your kegerator. Use white lightening caulk to fill the seams. Over lap the ends with 2 " wide tape. Spray with a krylon appliance paint that matches your appliance color. Mount the door back onto your kegerator. Drill a hole for you CO2 line and one for each cornie you can squeze in there. One, maybe two, depends on how big you make your collar. You can use your freezer shelf to store your hopps in a well sealed plastic bag.
Hope this helps.