If you want to extend an Ext.NET class or make a new ExtJS component support, you can just extend Ext.NET corresponding classes. There are several forum threads covering this subject here, a good starting point would be this thread:
- How to extend Ext.NET components
Although it is a discussion for Ext.NET 1, it should have most of the needed aspects covered at least. It would help extending and understanding how it works if you have Ext.NET source code -- which is available for premium subscription users.
If you give a google search on forums you should find several other threads showing different aspects on how to extend and create new classes from client-side customizations. A good google search would be "site:forums.ext.net extend class".
I hope this helps!