I make all my 9 x 18 Mak. brass from once fired 9mm brass,flare the case with the case mouth expansion die then run it through the 9 x 18 FL resizing die and them trim to .708 to .710" depending on which manual your using or just use a old once fired Russian steel case as a setup gage for your trimmer. I use the 9mm pilot in my Lyman Universal trimmer with a power adapter to make short work of the trimming.
One of my older Speer reloading manuals used the same process to convert 9 mm cases to 9 x 18 Mak. way before commercial Mak. brass or boxer primer ammo was available.