Passing waves for user defined fit function

Is it possible? and How?
No, AFAIK this is not possible. See DisplayHelpTopic "User-Defined Fitting Function: Detailed Description" for the details.

Is the standard solution a la
Function F(w, x) : FitFunc
    WAVE w
        Variable x

        Wave root:myWave // additional wave

    <body of function>
    <return statement>
End

not viable for you?
It is possible using a structure fit function. But they are difficult to use- structures can only exist inside a user-defined function, so FuncFit has to be called from within a driver function.

Otherwise, use the solution proposed by thomas_braun.

John Weeks
WaveMetrics, Inc.
support@wavemetrics.com