I think that you could emulate this, but not call it directly. Since you cannot modify a running VI, you cannot configure an Express VI while it is part of a running VI. Taht being said, you can get the code that the Express VI calls when it launches its configuration page and do what it does. The configuration dialog is in LabVIEW/VI.lib/Express/Express Analysis/SpectralConfig.llb and is the VI called Configure Spectral Measurements.vi. This VI can be roughly broken into three parts: Initialization, UI, Set Settings. The initialization part gets the inforamtion from the Express VI and load it into the configuration VI. The main while loop does the UI interaction that you are looking for. The Set Settings part writes into the the Express VI the configuration from the configuration VI.
You can grab the main while loop and use it as your UI on your front panel. You then will be doing interactivly what the Express VI does interactivly. It will take some work, but I think it should be fairly straight-forward.
Hope that this helps,
Bob