Bug 15832 - Tasks: Need ability to assign tasks to someone else is an RFE which also has a workaround for what you need. Basically right now you need to access the other user's Task list as a Manager and directly place the task there.
(See my comment #10 there.)
Currently, implementing of this as a feature within ZWC is being put off until IronMaiden, that is, two major versions after ZCS 6. Implementing in ZCO should probably be a separate RFE, so I just created it:
Bug 50281 - RFE: assign tasks using ZCO Please consider voting on both.