I'm creating a dynamic workspace which reads Excel sheet as a configuration and extracts data from a Feature Service. Configuration file looks like this:
FME workspace looks like this:
Feature reader reads every row in configuration file and extracts data for the listed Feature services. 
It outputs everything through the generic port.
In the configuration file I have field mapping information where attribute fields in a feature services are mapped with the output fields I need:
FME evaluates and extracts attributes using such logic:
It works well and I'm getting what I need:
But I have a question what to do when I have some more specific logic. For example, I have cases when I need to concatenate the fields into one field. How to pass this to FME to evaluate such expression?
I can't say that I like it, as it is not very dynamic, but in any case it is more or less what I need. In configuration file I have added such description of the 
And in the FME I have added additional logic to process the data according to the description in the configuration.
Configuration line in a file looks like this:
In PythonCaller it looks like this: