ONIX is the metadata format used by publishers (just as MARC is used by libraries).
You can convert ONIX files into MARC XML for ingest into NNELS. However, at this time, there doesn't exist a XSL for converting ONIX 3.0 to MARC XML. As such, we can only work with ONIX 2.1 files.
To convert an ONIX 2.1 file to a MARC XML file, the first step is to open up the file to check if it's in ONIX "short" or "reference" form.
ONIX short form contains tags that are only machine-readable, while ONIX reference form contains tags that are human-readable or plain language. See Editeur for an example.
If the ONIX file is in reference form, you'll need to first convert it to short form before you can convert it to MARC XML.
If you're ambitious and would like to create an ONIX 3.0 to MARC XML xsl, OCLC has published a Crosswalk from ONIX Version 3.0 for Books to MARC 21 that can offer guidance.