I have few Windows servers in my DMZ which are not part of my AD domain, how can i discover the users on those servers via IBM Security Secret Server?

Out of the box, you can only run discovery on windows servers that are a part of a domain. If your servers are not part of an AD, discovery will not be able to scan them. It is possible to build a custom discovery scanner to scan these servers. A business partner or IBM Services should be able to help you.

I am business partner and trying to get my hands on such documentation or guide from which i can create a custom discovery scanner for windows workgroup server(s)

The only documentation we have is here: https://www.ibm.com/support/pages/extensible-discovery-overview

Unfortunately we do not have any examples or guides specific to what you are looking for.