user registration requests

I would like to develop an alternative user registration system whereby an external user requests to join an app.  The idea is as follows:


We have a custom developed mobile app.  After the app has been downloaded, a person would request to register as a user.  The mobile app is linked to a single quickbase app through the API.

Currently we have to create the user and send them a link where they would register and then create username/password and then enter those details, but I would like it to be the other way around.  

Users can register, it then comes to us as a request and we then approve the user to be loaded on the quickbase app.

I see a lot of complications here, but I have the following idea:

1. The mobile app, when downloaded is automatically logged into quickbase on a demo user (this will allow API calls to be made to the database)
2. The user will then go through a registration request page (on the mobile app).  Once completed, the data (Name,e-mail, user etc) will be added to a different table in the quickbase app called (pending user registrations).
3. We as admins can then approve/deny the request.
4. On Approval we can automatically create a new user in quickbase with the supplied details.  
5. The person will then receive the normal link that quickbase automatically sends to e-mail
6. He will then register his password normally.
7. He can then login to his new account on our app.

So point 1-3 is a workaround to get the applicants details
Point 4 is the unknown here (can this be done)
point 5-7 is the standard quickbase registration


Apologies if i am not making sense here, but I did try explain as best as possible.