use a loop to copy from source to destination arrays then null-terminate the destination array. I'm not going to write it for you, you need to figure this out yourself so that you understand it better. Read about loops in your text book or an online tutorial.
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 ...