Probably not upgradeable
Given the sheer amount of custom coding that was put into their original implementation I'm not surprised they're still running it and haven't been able to replace it in 15 years. We never saw anyone else who did things the way they did.
Hint: Do not attempt to rewrite any complex piece of software like SAP to make it do basic HR and ordering stuff in a way which is unique to your personal twisted way of working. Especially when your implementation partner and even SAP themselves advise against it.
Anon 'cos of having some association with this monstrosity early in my SAP career. I still feel sorry for the good people who quit rather than have to face dealing with them. There were some of the most bitter and twisted creatures I've ever had the misfortune to work with in that place, and it seems their management have not changed.