I'm using an ArcGIS Feature reader to read a layer with two tables. After reading the two tables, I join them. I add and remove many attributes. The last step of the workspace is to use an ArcGIS Feature writer to populate a layer. 

In the past when I've done this, the ArcGIS Feature writer created the ArcGIS feature service. Can I write to an existing feature service?

ArcGIS Online (AGOL) field names must be 31 characters or less and not have spaces or other special characters (underscores are fines). Display names are also supported which can be longer than 31 characters and have spaces. 

Ideally, I'd like to specify a list of attributes with data types for the feature service. 

Questions:

Can I create an FME workspace to write to an existing ArcGIS feature service?

This is an ArcGIS question, but I'd like to easily create ArcGIS feature service based on a list of field names and data types. I think that I can do this in Python, but I don't know Python.