If the page holding the WebDock is in the same ASP.NET application as the page in the IFrame, then it will be best for them to share info with session variables.
Let's call the page holding the WebDock 'Page A', and the exsiting app 'Page B'.
We'd suggest you to move the user login to 'Page A'. Upon successful login, login info as well as other info to share will be saved in session variables. And then the main content panel will load 'Page B' in an IFrame. 'Page B' should be able to share the same session as 'Page A'.
Note that the above way could only share info when the pages refresh themselves from the server. But I think you can have client-side JavaScript to fire a trigger from Page A to refresh Page B and vice versa.
Hope this could answer your query.
Regards
Technical Support
ZettaCube Limited