There have been discussions on this in the past (sorry, I don’t have links right now). While there’s no standard that stops you adding new files, there’s a risk that if you do, then a future standard might claim the name you used for a different purpose. Ideally, we should have reserved something like a tool subdirectory for application use, but we didn’t.
As a practical approach, I’d be inclined to suggest putting all of the files you want to add in a subdirectory named (say) _typeshed, to minimise the risk of collisions. And then, if a standard does emerge, all you’d have to do is rename the one directory.