Hi Mario,
As far as we know the new version of the QuickBooks SDK, which hasn't been released yet by Intuit, does not support access to the leads center. This means that no developer will be able to add leads to QuickBooks using the SDK.
When it comes to accessing additional features released in QB 2013, such as adding multiple contacts per customer, the SDK is still in beta so once that is released out of beta, we'll begin to implement new features into Method.
- Ashur