At this time, we can support billing accounts in the following manner:
1) Have your billing contact sign up for a new Pushy account
2) Have him/her enter payment details to upgrade the account to Pushy Pro
3) In your own account, transfer your app to the new billing account
4) Have your billing contact add you as an App Collaborator
Billing for this app will now be tied to the new billing account, while granting your own account read/write access.