I would most likely set up an approval process.  Technically, everything in Altiris Helpdesk is an Incident from the moment of creation, but that's not to say you can't leverage the "Type" field to differentiate between Service Requests and Incidents for reporting purposes.  I would probably use the "Status" field to track the approval process while it is a Service Request, and the incident status when it is an Incident.  A lot of this can be automated using Incident Rules and custom e-mail templates.

I agree with bgreen.  If there is some sort of critical business reason to not use helpdesk then you could always look at getting Workflow solution.