How can I check to see if a user already has a username?

Basically I want a form user to be able to enter a username when they check out.

the names are stored in a google spreadsheet.

If they do not  enter a user name that is in the spreadsheet I don't want them to be able to continue without signing up on another form which will put them in that google spreadsheet and then they can enter that username to continue.

is this possible without manually adding the names in conditions each time a new user signs up.

Unfortunately, it is not possible to read the data from external sources based on an input field. The only possible way to validate the data manually by setting up the conditions. 

It may be possible by embedding the form using full source code and adding additional programming code to lookup the username from the spreadsheet. You may hire a programmer from Upwork or Freelancer to achieve your requirement.