Yes, I just did a quick test.(FYI - I am in SCM 7.0)
1. Key figure Infoobject do NOT have "Fixed key figure"
2. Above Key figure in Planning Area has type "Fixable Key Figure"
3. In Planning Book, Entered the number in above key figure. Right Click on it, Selected "Fix Cell(s)" option. Lock Icon appeared next to the cell where number was entered.
It should work in your case as well. Please check
Thank you
Satish Waghmare