Membership - custom members registration


I am working on a website using the Membership plugin, so far it is a nice plugin to use, I need to the Events+ plugin and the Membership plugin in order to set multiple prices per events,

So my question is the following, is it possible to assign a subscription to an existing member using a custom SQL query (or simplier) ?

My goal is to create a register form where the users can create an account and select a subscription without paying anything, I know I could use the built-in registration form, but I want to change this form and I cannot do it without altering the plugin itself, please correct me if I'm wrong here.

Thank you for your help.