Add items to the "Open with" list in Windows 8

The answer is ridiculously simple - just make sure your application has disk:\path_to\your_app.exe.ApplicationCompany string with any meaning e.g. Soopa-troopa soft in [HKEY_CLASSES_ROOT\Local Settings\Software\Microsoft\Windows\Shell\MuiCache].

After creating this registry key open desired file with your application and it will appear in Open with list.

There is a program mentioned in another Question. Maybe it will do the trick. It's called 'Default Programs Editor'. The changelog says its old though. Last update from 2010. There is also an answers which dives deep into the topic.