Thanks D'souza & Ajay.
I think the Ajay's alternate solution is quite useful.
I will use user exit COOPA0003 to add custom fields for project IO.
For project costs that is directly related to a consultant, the ID of the consultant will be entered in the custom field.
For normal expenses that is not related to any project, it will be charged to the individual employee IO.
If manager wants to see the total project costs, a SAP standard report will be sufficient.
If manager wants to see the total expenses of the consultant, both project and non-project related, a customized report will be needed.
Think the above should be able to work.
Sebastian