Projects in OpenERP V5 Part 1

Code coming from the project module:

class project_work(osv.osv):
    _name = "project.task.work"
    _description = "Task Work"
    _columns = {
        'name': fields.char('Work summary', size=128),
        'date': fields.datetime('Date'),
        'task_id': fields.many2one('project.task', 'Task', ondelete='cascade', required=True),
        'hours': fields.float('Time Spent'),
        'user_id': fields.many2one('res.users', 'Done by', required=True),
    }

...

In this code, you can see that in the project work you have a user_id but It will be more interesting to link the employee (from the hr module).

Here is the problem:
You have a company with a lot of external consultants, you don't want them as users but you need to encode the time spent by these external consultants on your project to invoice your customer...