For more background, please see my forum post on this issue:
https://forums.autodesk.com/t5/upchain-forum/no-way-to-globally-attach-linked-documents-to-parts/td-...
It appears that Upchain currently does not support adding Documents that exist outside of a project to BOM Items within that project. I am able to Link these documents to the project by using the "Share" feature, which then makes the document available to the project... however I cannot go a step further by adding these Linked documents to BOM items.
A Global scope for documents is crucial for my PLM usage. Being able to reference documents globally prevents duplicates and confusion.