You can't, exactly.
You need your own 10mm casing and bullet heads to fit the size of 10mm. You're only taking apart the .38 bullets for the powder.
So, you'd have to pick up every spent 10mm casing you fire, just so could convert .38 bullets to 10mm.
Realistically, no one would ever do this. Plus I have no problem finding 10mm rounds, and I use the Deliverer all the time, and its a 10mm gun. I have well over a thousand for it, and i've only bought about 100 rounds. Try having more than 1 main gun, that usually helps.