The process usually needed Nimoy to be in the make-up chair for about 45 minutes ... The process was eventually streamlined, and Spock’s ears were made of latex and cranked out on an assembly ...

all someone needs to pass as an "alien" is a little bit of make-up on an otherwise human face. (In Vulcans' case, it's pointed, elvish ears.) Spock's home planet is also named Vulcan after its people.