Hello Lauren
Unfortunately, it is not possible to do this with one form field. You can use the list field that you have currently created for you web content record in the scenarios where you want the person submitting the form to set the source value themselves, but you will need to create a new hidden-type form field to use on the form where you don't want to set the source value yourself. If you have not used hidden fields previously, here is an article that will walk you through how to set it up: http://help.clickdimensions.com/mapping-to-a-leads-topic-field-hidden-form-fields/