[OJS] Including additional metadata within Native XML imports

Describe the issue or problem
I’m working on a big migration from a custom platform towards OJS and I’m working on some plugins which add some custom metadata to submissions for one or two edge cases in order to accommodate the current featureset from their bespoke platform.

However, in this migration I’m creating a Native XML import file for OJS to digest and to pull all of the journal information into one place. I’m getting a little stuck when looking deeper into this and how I can possibly ask for a submission to include some additional metadata to be imported (if this is at all possible at this point?)

What application are you using?
OJS 3.3.0-4