Welcome to the forums.
This feature is available in OXCE+, a fork of the vanilla ipenxcom. It contains some additional features, but is a couple month behind the standard code, so may lack the most recent additions.
So to use this feature you should switch to OXCE+, or maybe convince developers to include it in vanilla openxcom.
EDIT: I was informed by Meridian that this feature is already available in vanilla, and you are probably using some old or obscure version, like the one provided with Soldier Diaries.