Google announced in the Google AdWords Help forums that they have updated the Feed Rules to give you more flexibility. Feed Rules launched in April 2016 and was designed as “a self-service tool to help you transform your existing data from your inventory management systems into the format accepted by Google Shopping.”

Now, with Feed Rules, Google Merchants can do even more:

Partially change or update existing values. For example, you can update specific values of the color attribute while keeping other existing color values intact.
Create new values by combining static values and/or values from different attributes in your feed. For example, you can add size type values to your product title values.
Extract values from other attribute values. For example, you can populate a color value by searching in the description attribute for pre-selected values (blue, green, red, yellow) that will be suitable for the color attribute.

There is a lot more detail around this over here where explained:

Forum discussion at Google AdWords Help.