Short answer, you don't do this. Usually when you want to get values from a thread you pass it a queue, and have a your main thread get values from the same queue.
A couple other things: you're calling join(), which will block until the thread is finished making the while loop directly after it unnecessary, and don't use "str" as a variable name since it's already the name of a builtin function.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...