Mapping custom fields from one module to another

Mapping custom fields from one module to another

I have a custom field, "Subscription Period" that appears as a required field in every Opportunity (Potential).  I want that field to appear on any Quotes derived from that Opportunity (and have created a custom field of the same name in Quotes for that purpose).  What are my (non-programmable) options for linking the two?  I know that the field will be mapped to an Invoice or Sales Order going forward, but how do I "back map" it to the original Opportunity?  The solution must be obvious, but I'm not seeing it.  Any help appreciated.