This is difficult. It can be scripted, I think using the Explorer OM.
You can also use this tool (it won the Biztalk Design Contest in 2004): http://www.gotdotnet.com/workspaces/workspace.aspx?id=992ca223-553c-475a-ac87-da7ae2c9016a
Also, Todd Uhl’s blog has some deployment scripts for working with parties: http://geekswithblogs.net/toddu/archive/2004/11/09/14602.aspx
Hope this helps.