One easy way to do this is to create attributes using the import wizard. Check
this. Though the approach is described for CRM 2011 the same can be used for 2013.
1. Export sample data from lead(so you have an excel with the fields same as that of lead). Add any additional fields required for opportunity.
2. Import the file and create new fields while mapping the fields
Fields will be created with this. You'll have to take of the scripts after that.
Hope this helps!
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