Hi John,
For your first requirement i.e., addition to an asset, you can maintain in AO25 the year and period combination for many years by simply specifying the year and the periods along with the total units column, leaving the period unit column blank for future periods, so when ever you make additions to that asset you can add to the relevant period the number of units to the total unit column and the period units to the period units column. This way you can continue with the same depreciation key for many years.
In case if you are acquiring a new asset which needs to be maintained separately then you have to create a new depreciation key.
You can refer to the following link for clarification.
Hope this helps you.
Regards,
Divate Rama mohan