You can reuse a DITA topic simply by including it in multiple maps (or submaps)
using the following procedure:
To create a new map, select File > New or click the New button on the toolbar, go to Framework templates > DITA Map, and choose the appropriate type of map.
Add existing topics to the new map by dragging and dropping them from the
Project view or the file system (or right-click the map icon, or
on a topic already in the map, and select Append child or
Insert After).
If your topics use key references, set up the appropriate key definitions in your new
map. You can set the keys when you add the topics, or afterwards by right-clicking the
topic in the DITA Maps Manager, selecting Edit Properties, and defining
them in the Keys tab.
If you want to define relationships between topics, other than those defined in the
topics themselves, add a relationship table
to your map or to a separate map linked to your main map.
When you have finished adding topics, check that your map is complete and that all
topic links and keys resolve correctly. To do this validation, click the Validate and Check for
Completeness action on the toolbar in the DITA Maps
Manager.
New or click the 
Edit Properties, and defining
them in the Keys tab.
Validate and Check for
Completeness action on the toolbar in the DITA Maps
Manager.