Can evaporated milk be converted to "regular" milk? I've got a recipe that calls for 2 1/2 cups whole milk, but I don't drink whole milk all that often and if I buy any it'll just end up spoiling in the fridge before I can use it all.

Canned evaporated milk, on the other hand, lasts a lot longer and from what I understand its just regular milk with ~60% of the water taken out of it. With that in mind, would I be able to convert evaporated milk into "regular" milk for use in cooking just by adding an equal amount of water?
Yes, you could certainly reconstitute evaporated milk by adding water. 

The outcome would give you similar but not identical results as fresh milk, due to the change in flavor the milk underwent while being cooked down to the evaporated state:  it will retain the slightly cooked, caramelized flavor.  This may or many not have a significant effect on a given recipe.

Milk also freezes, and still is suitable for most cooking purposes, should you choose to do so.

Finally, if you use milk very rarely, unless you are making a dairy dominated recipe like custard or cream soup, you may not have to use milk at all.  Water or another liquid might perform similarly or well enough&mdash;it depends on the specific application.