ULX has no default method to add many at once.
Though Ulib's users.txt file is easy to see what it does, the simplest method would be to use Gmod's normal user.txt file and format.
Issue is, I don't remember if we(ULX) will store them in ULX own user files, so that you could remove them from Gmod's, and change/promote them later using only ULX.
(I know we load Gmod's at startup)
As for the amount, yeah, just crazy, but, to each their own.
Gmod users.txt file is this format, in gmod_server_path/settings/users.txt
"Users"
{
"<groupname01>"
{
"<playername01" "STEAM_0:1:###1",
"<playername02" "STEAM_0:1:###2",
}
"<groupname02>"
{
"<playername03" "STEAM_0:1:###3",
"<playername04" "STEAM_0:1:###4",
}
}