You can reuse a DITA topic simply be including it in more than one map:
Create a new map by clicking the New... document button and
select Framework templates > DITA Map and then 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 on 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. Set the keys when you add the topics, or afterwards by right-clicking on the topic
and select Edit Properties....
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.