Kastytis, I can provide a couple of simple ideas as a start. (1) have you tried using fixed duration tasks and entering your work resource with 0 hours? (2) if all you are trying to do is to assign the resource responsible for the activity, you could try using a task custom field and assigning the resource there. Granted it does not roll up to the resource over all his/her tasks but that depends on how you are using Project Server. Thoughts?