I would like to estimate durations for work packages level - not on individual task / activity level. The reason behind this, the activities are duration less action plan items that are simply to be checked off while the work package (a group of action items) can be planned to be worked on in the specific order as well as with dependencies.
Alternatively, if it is possible to setup sub-tasks inside (not as child tasks but as a collection of checklist items), then the action plan can migrate inside the work package entry and that entry will no longer be a parent - duration problem solved.
However, it seems I cannot define the duration of that level unless I put some fake task "Working on action items of this group" child task or fake durations of action items.