Customize author migration

Hi

I am new to copybara, our project requests to process author by their domain, like xxx@domainA.com and yyy@domainB.com. Need to passthrough all users from domainA, but for Domain B, want to alter them as default one.

I have two questions:
1. can I create a customized function to assign to authoring inside workflow?
2. how can I get the current commit author info to pass to this function?

if there is any example, that will be great appreciated.