You can import an existing map as a set of topics for the current map. The imported map's central topic will become a subtopic of the currently selected topic.
Do one of the following:
Select a topic, then on the Insert tab, in the Map group, click Map.
Right-click a topic on the current map., click Insert, then click Map,
Choose the map you want to import.
If you only want to import part of a map, you can open it and copy the topics, then paste them into the current map, or simply import the whole map and delete the unwanted topics.