There's no way to achieve exactly what you described. Apps for Business shouldn't have anything to do with it -- two constraints are relevant here:
A given email address can only be associated with one AdWords account.
An existing AdWords account can't be converted to an MCC account.
So you have two options, depending on what's important to you:
Simply create your MCC account with a new email address (say,
paul.mcc@yourcompany.com) and link the existing AdWords account to it.
Associate the existing AdWords account with a new email address and then associate the MCC account you create with the existing email address.
Option #2 is complicated, so you should only bother with it if you really care about your existing email address being associated with the parent MCC account. Assuming your current AdWords account is associated with
paul@yourcompany.com, here's the step-by-step to achieve this (I'm pretty sure this is accurate, though you should verify this yourself):
Create a new email address -- say,
paul.adwords@yourcompany.com.
Add
paul.adwords@yourcompany.com as an administrative user on the existing AdWords account. This will require creating a password for logging into AdWords with the paul.adwords@ login, which should probably be different from the password for the paul@ login.
Logged in as
paul.adwords@yourcompany.com, revoke access to the account from
paul@yourcompany.com.
Now create your MCC account using
paul@yourcompany.com, as though that email address was never associated with any AdWords account.
Link your AdWords account with your new MCC account.
