Samurai came out in Xanathar's Guide to Everything. But it doesn't mention anything about katanas. The 5e system isn't finely detailed enough to differentiate it from a long sword. As for why the compendium cannot be added to by the typical user? It's very complex. It's a system that takes considerable training to use without breaking. Roll20 has stated many times that they would like to have a user-expandable compendium (and by inference, Charactermancer), but there's no way to make it user-friendly. You can develop such things, of course, if you have sufficient programming skill, but then you are usually talking about building a marketplace presence. Kobold Press for instance, does this. There are ways to automate adding specific abilities to characters that use Mod scripts, but that requires a Pro subscription.