I have a Craft 2 project which is using an older version of the FeedMe plugin 1.x. In order to use a specific XML feed this site uses, a custom XML feed service was implemented with a custom feed type available to select from the feed type option in Craft FeedMe back-end, but this involved directly editing the FeedMe plugin files. 

The reason for this is because a specific authentication mechanism is required before being able to interact with the feed in the form of a unique token passed by the Authorization HTTP header obtained from a separate auth endpoint first. This is then added to the feed request via curl, so its authenticated and returns data.

The current implementation added an additional service within `feedme/services/` which extends `FeedMe_FeedXMLService` to handle the token authorisation part and added an additional enum type, record type, model type. In theory could a separate plugin be created that provides the service without having to modify the core plugin? Or could the approach be different given the changes with the 2.x version of the plugin?

It looks v2.x has changed quite a bit looking at the core plugin files with potentially more options for customisation, but I'm not sure if the implementation we have is possible to move to 2.x easily.