Sorry, if it is prebent maybe it is a replacement type stem that is ready to use as it is, instead of the stem "blank" kind of things I am used to which are made to be worked on prior to use. So, never mind ...
If I had to open a prebent, though, I would heat it up with a heatgun or candle and bend it back to flat (with a pipecleaner through the airway to keep it from collapsing), drill it, and then bend it back.