Migrate multi-value paragraph field
This is not directly an answer, but it is also too long for a comment :)

When I migrated something that wasn't actually paragraphs into paragraphs, I just wrote a custom process plugin that created paragraphs on the fly, then I didn't have to bother about trying to figure out some kind of mapping/made up ID's. What you're doing definitely seems like a bad idea, as 1010 could either be delta 10 from node 10 or delta 0 from node 101. 

Put two classes I used for that into a gist: https://gist.github.com/Berdir/8a650a29a670fe283589f8946354086c. 

It is both a bit hacky and a fairly complicated as I needed to support importing multiple translations for separate nodes in 6.x to a single node with a single paragraph with translations in 8.x.

Also created a to-do for me to write a blog post about this. ;)