Tracking plan items by work hours ?

The RTC agile planning component distinguishes plan items vs. work items. Unlike work items which can have attributes 'estimated' and 'time spent' used to determine the 'progress', plan items have a complexity attribute, e.g. 'story points'.
A customer now wants the plan items to be tracked by work hours as well. The value might be accumulated by the values (e.g. 'estimated') of the child work items.

Unfortunately neither 'estimated' nor 'time spent' can be selected when configuring the complexity attribute in the project area.

Any ideas how to achieve the desired behaviour?
Andreas