Hi
For additional acquisition in same year, you can just do F-90/Migo again in the same year / same asset
For additional acquisition in next year after the previous year is closed , you must do Abnan I.e. Post capitalization
that's the right process because it catches up the whole dep. Also don't use smoothing in oayr
you can also raise oss message under component xx-csc-IT as you say it is alegal requirement. Sap would tell you what have they designed to meet this requirement
Ajay M