You can use external assemblies in SSIS Script Tasks and Components but they have to be strong-named and in the GAC. Here are a couple of older blog posts about it, but I don't think anything has changed:

http://consultingblogs.emc.com/jamiethomson/archive/2005/11/02/SSIS_3A00_-VSA-requires-DLLs-to-be-in-the-Microsoft.Net-folder-_2800_but-not-all-the-time_2900_.aspx

http://microsoft-ssis.blogspot.com/2011/05/referencing-custom-assembly-inside.html