Get user email address as Global variable in workflow designer

Is it possible to get the email address from a useraccount as a workflow variable?

Yes this is possible using the endpoint GET Organization/UserByID?userId={userId} of the web service linked to Docuware Platform API 

create workflow variable userId_U of type 'user of a group' and assign appropriate use account your want retrieve the mail address
in the request, select the query tab and map the userId field to the workflow variable UserId_U
in the Assign data, select the JSON path (once you've sent a request) $.EMail to workflow variable of type text

Hope this helps...