Is it possible to add a field to the Membership Plugin

I want a gated site for regular visitors and wholesalers. Wholesalers are defined as people who have a state tax ID. I need to have the tax ID field as part of the account registration form and keep the tax ID with the account information.

If there is a better plugin to use than this or another way to achieve registering for an account with a state tax id please let me know.

Thanks. Kim