Back in the FS9 days, Grumman pitched a tantrum when anyone released any version of their planes - even freeware.
I would suggest looking up the maker’s website and finding their Contact page. (Example) Send an email to the main address listed and in the subject line, include something like “Attention Public Relations” or similar. Explain in detail what it is you want to do, especially whether you want to make it freeware or payware.
Worst case is they say don’t do it. Best case is they decide that what you’re doing will be free publicity for their company and they take steps to help you make the best addon you can.