Hi Xceller
Once you convert to cube formulas, there is no way to go back to a pivot table (and hence, the the field list). That is, unless you start a pivot table from zero again. Having said that, you *can* still use pivot table generated objects against cube formulas, like slicers and filters.
Javier Guillen
Senior BI Consultant
PowerPivot Blog:
http://javierguillen.wordpress.com
Twitter: @javiguillen