Want to impersonate a user that is referenced in a field from a previous step, where I opened a form

Hi Team,

I am working on ATF in Zurich release for an GRC module.

I want to impersonate a user that is referenced in a field from a previous step, where I had opened a form using ATF. 

Please find the attached screenshot for the same.

Also, please assist how can I create ATF test case for assessments in GRC module.

Thanks in advance.