sed output to be captured back by procmail

Greetings ... !

I'm am seeking the correct approach and syntax for the following
situation...  

1st:  I would like a recipe to detect arriving mail from a
      particular source.

2nd:  If conditions match, cause the header/body to be piped and
      edited by sed. 

3rd:  Cause the output from sed to be piped (and|or) captured
      back by procmail to be used in additional recipes, maybe
      within a nesting block, before final deliver(y|ies).

As requested, Thanks in advance!  :-)
-Bob