How to add a method to a class / how to make the runtime think a class contains a method?

Question

I have an app that I try to make work on my phone, but on my phone I don't have some of the vendor-libs needed to run the app.

I created a Magisk+Xposed module that patches in the missing classes, but it turns out that the vendor added stuff to core android classes (like com.android.View).

I successfully patched the ClassLoaders, made reflection work, but some calls to vendor-added methods are direct, so the app crashes.

How do I add that methods to the classes, or how can I tell the runtime to look for these methods elsewhere?