Is it possible to use external editor instead of internal one which has limited features?
As far as I know it's not possible to use an external editor interactively within MindManager as a replacement for the existing topic notes editor. The ability to do so and/or the replacement of the built-in editor have both been requested a number of times. I can only hope this is on the MindManager to-do list of improvements.
For the time being you could experiment with using an external editor like Word and pasting the resulting document in topic notes. You may lose some features but a surprising amount of the original document's formatting and layout will be retained, including some aspects which are more difficult or impossible to apply if you create the material entirely in topic notes. Provided you are making only minor changes you can also edit the imported text in topic notes and retain the imported formatting.
If you're looking to use a different text editor for taking notes instead of the default Notes app on your device, there are a few options you can consider: