Associating a cost for a resource but not always applying it

Hi,

I was wondering if anyone could answer this question for me.  I have assigned a cost to a resource in Project Online and can view the cost reports to show how much the resource has cost the project so far etc. 

Is there a way to assign a resource to a task but because it may be a non chargeable task, we don't take the cost of the resource for that task into account?  Or would I just need to create a separate resource e.g. John Smith (non chargeable). 

Thanks,

Gillian