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Yes, it is possible.  In the first action of your process you can pass as an IN parameter the macro "Request.Relative Id" to the form.  The first relative ID for any process is 1.

Note that each request's unique Request number is still the Request Id that WorkflowGen generates automatically and that is predominantly visible on the user portal.  By making the Relative Id an additional DATA element of your process you can display this Relative Id as well in the portal in the same way other process data can be displayed.