I have a large literature collection that I want to import into Paperpile. I can generate any (documented) input format with any modifications, representing my file locations, folders, tags, and keywords in whatever way may be required.
I would like to achieve the following:
(a) link PDF files from my local storage to their respective records, regardless of DOI
(b) convert my tags into Paperpile labels upon import
© convert my folders into Paperpile folders
(d) have my keywords become searchable in Paperpile
How should I represent my data for loading into Paperpile? (the answer could be e.g. “use BibTex with the following custom fields:…”)
If only some of these objectives can be achieved, fine. Direct use of, or simple modifications to, common formats would obviously be preferable. Polishing the data through an API after loading is fine. Manual modifications to data or transitive loading via Zotero etc is not fine 
Thanks.
Dmitri