Recording who converted estimate to sales order.

Hi All,

For reporting purposes we're interested in capturing which associates are converting estimates to sales orders. This would potentially feed into KPI's. Would we require a script or workflow to capture this data? At present I have tried to access this data on a saved search but it is unclear if this is available, even on system notes. So far the best I've found is the 'Created By' field. But this can be flawed as one associate could create the Estimate and a completely different one could have actually hit the Sales Order button. 

Any ideas would be useful.