How can I copy data in a field from tab to tab?

How can I copy data in a field from tab to tab?

When I click CONVERT on a lead, the lead it sent to the "SALES" tab.  (I renamed POTENTIALS)  
 
The SALES (Potnetials) tab has custom fields that I need the content displayed on the ACCOUNTS tab. 
 
 
How can I do this?  I tried a lookup, but I cant look at fields in other tbas with that.