sometimes I just want to add a few extra fields to the type's children view, but if I write a regular type synthetic class, it will show only my new added fields, how can it have those origin fields included automatically without just re-adding them ?
There isn't an option to "type synthetic" to augment the current set of children. Feel free to write a enhancement request for this with http://bugs.llvm.org.
For now, you have to add all the children by hand. Since synthetic child providers are Python classes, it shouldn't be hard to make an "echo" provider class that just reports all the children. Keep that around and then every time you want to add a few fields to some type, make a provider that derives from the "echo" provider, and add the new fields onto it.