Donna, I'm not sure if you plan on getting the Sales Enablement tools but if you do, when you install Discover, for some reason it will create a new Marketing Activities object (which we then found out that we would need to update all of our external calls to point to the new Marketing Activities object). The good news is, those activity records placed in the new object will be copied over to a contact during teh conversion process.
Even if you don't buy licenses for Discover, you may be able to install the app into SFDC, remap your External calls to the new object, delete the old marketing activities object. I would vet that with others on your team but I think that could be a workaround if you can't get anyone to write the trigger.