Mapping custom attributes to Google product feed attribute

Hi.

We have 4800 products in our store, all of which already have custom attributes with the product Brand, GTIN, colour etc.

I can see that the plugin has included all new fields of its own for these attributes to be included in the feed.

Is there a way to map the existing attributes to google feed attributes?
Not only will this be a big job to duplicate this information into the plugin fields, storing the data in two separate places isn’t ideal.

Many thanks in advance.