But as I said, any content of a textBox is already a string, therefor is a text value not an integer... so you should just be able to call textBoxName.Text to get your string value as it seems your .dll has already pre-converted the int32 value to a string in order to populate it into the textbox in the first place.
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 ...