I want to do exactly what Hal has described above. Subscribing to post and hoping for more info.
(Side note: I found this article:
https://community.marketo.com/MarketoArticle?id=kA050000000KzejCAC that shows you how to append data to a field. I tested it with a multi-select field and upon populating the field the first time, it does actually select one of my possible values, but the second and third (etc) time it basically creates a whole new value that isn't one of my values. Ie. if I go back into Salesforce, I can still add the values that I seem to have selected. Weird and not helpful. Can't search them properly in reports etc. in Salesforce.)