Manager - Direct Reports - New User

Hi Team. 

I am looking for some suggestions on how to overcome a little issue. 

We have a new user creation form that we use for creating what we call Vendor accounts. This just spits out a very basic account in the right OU , with the RBAC groups etc.  

We also have a new user creation form that we use to create a seperate admin account and again spits out an account in the correct OU , RBAC groups etc etc. 

So that we can keep track of who admin belongs to which user account we have been looking at using the Manager and Direct reports fields and yes this will pretty much do for linking the accounts. 

The question is. 

On the user creation form we use for the admin account , we were thinking of some way of being able to select via a drop down a list of all accounts in an OU and selecting the account which the admin account must be a direct report of.

Has anyonw done anything lise this or has a better way of doing it? 

Thanks in advance