Hire someone who knows what they're doing? (sorry, couldn't resist)
But really, you need to clarify exactly what you're requirement is for this. For instance, what's your definition of 'inactive user'? A user who's logged in but not doing anything? Or is it someone who's account is inactive in the database. No one could give you any meaningful response without a more thought out, coherent and detailed question.
How can I run a load test with inactive users for a wed based application - Interesting. I wonder which customer has made this request [img]/images/graemlins/smile.gif[/img] How about keep refreshing the page after login or before, whatever scenario you are executing.