You can find possible approach in the attached project. I subscribe for the KeyDown event of the Stackpanel which holds MaskedInput and TextBox. I check whether the original source is the currency input (the textbox in its controltemplate) and whether the pressed key is Tab. Let us know if this helps you move forward.
sender, KeyEventArgs e)
isMaskOriginalSource = e.OriginalSource
(e.Key == Key.Tab && isMaskOriginalSource)
the Telerik team