Unfortunately, this is not possible.
FlexPro only supports linear and nonlinear curve fitting for 2D curves but not for surfaces.
With the FPScript-function NonLinCurveFit you have the possibility to define your own userdefined models.
Example: You have three data series x,y,z and you want to approximate the data series z with following function:
f(z) = f(x,y) = a * x + b * y
See the FlexPro help (NonLinCurveFit) for more details.