As Radu Dumitriu closed my question as duplicate without any hint to the question I duplicated and without any hint to as solution - what is not very helpful at all - I repost the question here (why can't I just reopen my original question?)
I want to import data (from polarion) over csv, everything is fine so far, but i can not map the original issue id to any of the available read only custom fields. Regardless what type of read only field I configure, it does not show up on import mapping by the available fields. All other custom fields are there, but non of the read only fields.