Can you handle some simple PHP changes? Search these forums for unified_search, that’s a flag you need to turn on to make Search work for a new field.
For example
EDIT: note that (I think) this can be enabled for a module, or for a single field. You’ll have to check if it’s enabled for Opportunities module, which I expect it is, and whether only that particular field needs enabling. I am unsure about the details on how to use this.