If an item in unreal library is older than the unreal version of the project it is to be added to (e.g item in 5.0 into 5.1 project) , it should be possible to add, since (according documentation) the newer unreal version can read(and load) items from an older version.
Correct. Is there a question?
Why is this? Since 5.1 has no trouble reading stuff of 5.0 (and older), or these can be migrated to 5.1. Ok when stated there could (or will) be issues, I would like to be able to add 5.0 or older packages into 5.1, or 5.2.
To be clear, you can always ( mostly ) go up engine versions, just not down 