If you can clarify further what you are wanting to add, we may be able to point you toward existing functionality, or may consider this as an enhancement for future version if it is generally beneficial.
Otherwise, if you have PHP experience and want to take this on a strictly a local customization, start with this thread for background on modifying the submission process:
Also consider the general mapping of OJS requests to the code for form processing. I described a similar process here:
I presume you’ve already found most of the code here:
If you can post what you have tried, and what parts are and aren’t working, we can help to guide you.