NOTE: The ALTER DATABASE DATAFILE OFFLINE DROP command, is not meant to allow you to remove a datafile. What the command really means is that you are offlining the datafile with the intention of dropping the tablespace.
I m not sure if u can add the datafile so easily.
if not, then u need to export all the objects in the datafile,
drop the tablespace including contents,
remove the datafile physcially depending on the OS
re-create the tablespace, add the datafile and import the objects..