Thanks,
6 answers
I'm a bit late to the party, but wanted to confirm that standard fields cannot be mapped to 2 objects (nor can the standard field mapping be changed).
As CTU007 mentions, the best workaround is a trigger on either lead or oppty to update description on the newly created opportunity.
The steps - opportunity before insert:
1) Query for any converted leads where convertedopportunityId IN Trigger.new
2) Create a map of opportunity Id to Lead.Description
3) Loop through trigger.new and update opportunity.description with the restults from #2 if found