Getting parent records to be related to multiple child record

so I have a similar issue, just not the n:m...

We have a parent record (a Campaign) that may include multiple Opportunities (child table).  The Opportunities table is already populated (and synchronized with Salesforce)...so we aren't creating a child record here, just linking it.

I think I have done this in an application in the distant past, but cannot recall the most elegant way to accomplish the multi-select of the child records (or a search for that matter)