OK, let me get this out there. No you cannot own a Glock 18 or converted Glock fully automatic pistol in the US. Two exceptions: You are an FFL/SOT or a law enforcement agency. Those are the only two legal ways to get one in the US. Period. If you get an FFL/SOT so you can get a G18, you're in violation of federal law. FFL is a business license, not an "enhance your personal firearms collection" license.
Here's the why:
In 1934, the National Firearms Act restricted ownership of fully automatic weapons, among others. Basically a transfer tax and paperwork. In 1968, the Gun Control Act instituted the FFL system among other things also banned the ownership of imported machine guns imported after 1968. The 1986 Hughes Amendment (which didn't legally pass, but that's another topic) banned civilian ownership of machine guns of domestically made after 5/1/86. This created three types of fully automatic weapons.
Fully transferable: Machine guns made (and registered) prior to 1986 and/or imported before 1968. These are civilian legal.
Pre 86 Dealer Sample: Machine guns made prior to 1986 but imported after 1968. These are dealer only, do not require a demo letter, and are not civilian legal. A sole proprietorship SOT may transfer these to themselves at close of business, tax paid transfer, but be careful and do NOT get an SOT just to get pre-86 samples. You will be found out and jailed. The ATF doesn't <deleted> around with this stuff.
Post 86 Dealer Sample: These are machine guns made after May 1, 1986. These are only available to dealers with a demo letter from a LE agency. They are not civilian legal.
Now, where does the Glock 18 fit? The Glock pistol was introduced in the early 1980s. There were conversions done on G17s for R&D but these aren't legal since they, being made in the 1980s, would have been imported to the US after 1968 cutoff, so it's dealer only. The Glock 18 wasn't manufactured until 1987, so it's a post-86 machine gun and not civilian legal.
I want to dispel some myths. There is no such thing as a civilian legal G18 in the US. There is no such thing as a fully transferable G18 in the US. There is not one single G18 in the NFA registry that is able to be legally transferred to a civilian in the US. I don't care what you read on the Internet, heard from a friend of a friend's cousin's boss that heard from a guy who knows a guy. There is no legal means of a civilian in the US personally owning a Glock 18 or any converted fully automatic Glock. Period.