My best suggestion for this is to use Tags. 
1. You will have to apply a tag to prospects across different newsletters. E.g. "Newsletter" tag, or any name that will be helpful.
2. When sending your newsletter, select "People Directory" (sends to all people), then add a tag filter for "Newsletter". This filter checks for any prospects the people correspond to that have the "Newsletter" tag.

That way, you can send to people with the "Newsletter" (or any) tag that are scattered across different pipelines. 
Remember that you can bulk tag prospects by selecting multiple, then clicking the three dot menu in the green bar at the top, -> "add tags".

Looking forward, We'll also be soon adding a pipeline action which will allow you to add a tag automatically once a prospect enters a stage. 

Thank you!