If you click on the express VI after you have configured it and select show front panel LV will ask whether you want to convert it to a standard sub-VI.
If you do that you can easily modify it to highlight the input field you want the cursor to start in, change tab-order etc...
Off course that's not an ideal solution as you loose the express VI's special configuration features, but I believe that's the only way you can do it yourself (or...?)