We just uploaded an example that uses a sigmoidal fit, using the Fit with an arbitrary fit option. Go to File > On-line examples to download. This example comes from: Sigmoid fitting in Excel (Excel Solver Add-In): dx.doi.org/10.17504/protocols.io.78ihrue
We have thought about a more complete library for functions, but often different fields have different naming conventions. What you can do now is create your own function library.
Here is demo that shows a non-linear fitting example and how to create the library: https://www.youtube.com/watch?v=OoYrYiggxHY
Does help with your use case?