Changing or adding an activity to existing records

I want to add some existing records to an activity, and also change some wrongly attributed records to the correct activity (oops, butterfingers!)
I see from previous comments that it's not possible to do this via editing.  
My plan was to download a batch of records, delete them, and then upload the spreadsheet with the correct activity edited into it. 
However, at the step where I map the import columns to destination database fields, for 'Group title' (which is the heading for the activity names in my records) it says 'not imported', and I can't see any suitable option in the drop down list.  
Any suggestions?