B.A.
Is there any way to pump more often at work? As a general rule, the more you pump, the more milk you will produce and the less you will need to supplement.
Pumping while at home can also help increase your supply. Other things that can help increase supply are: eating oatmeal, eating kale (you can sneak finely chopped kale into just about everything! chop it fine and freeze it and throw it in spaghetti sauce, meatloaf, or whatever you eat often), and drinking certain herbal teas have helped many women increase their supply.
Those methods don't always work, but most times at least one does. Pumping on one side while nursing on the other (while at home, obviously!) helps many women with letdown and they get more pumped milk than if they were just pumping alone.
I know my answer probably isn't quite what you had in mind, but the less you have to supplement, the less you even have to deal with the transition to formula. You've also already gotten a good response regarding the transition itself :)
Best wishes!
~B.