After you pass through the stylesheet that extracts the payload from the SOAP envelope, you can add an XML script based rule. Your current context should be batch-response. That would match the condition, "if operation equals batch-response". So that's the first step in a rule.

That means that the xpath expression "." which represents the current context is batch-response, so if you do

if operation equals batch-response
append XML element, element name "field1", XPath expression ".",
append XML text, XPath expression "field1", string "blablabla"
append XML element, element name "field2", XPath expression ".",
append XML text, XPath expression "field2", string "abc simple as"
append XML element, element name "field3", XPath expression ".",

you should be able to accomplish your example.

BTW, I've developed and deployed a mature, fully featured Workday driver which supports all kinds of cool features like a highly automated configuration for workday setup, event driven push to IDM, bidirectional communications, workers and contingent workers as well as transitions between them, synchronize whatever attributes you wish including pictures, reference synchronization (locations, departments, contingent worker types, time zones, you name it) to roles and role assignments, effective dated transactions, and way more at several different clients (which doesn't rely on XSLT except for the one OOTB unmodified stylesheet that strips off or adds the SOAP envelope).