Thank you for your post!
We currently do not support connecting an existing Stamps.com account. You receive a free Stamps.com account with your ShippingEasy subscription.
Here are the steps for starting your free postage account: How to: Open a new USPS postage account for free
If you would like to be able to connect an existing postage account in the future, I recommend posting your request in our Feature Request forums
. Posting there puts your request in front of the Product Team and also allows you to receive an email when the enhancement has been prioritized and/or released.