Generally if you're using CDO you can use AddAttachment but the file has to already exist on the server.
Examples of objCDO.AddAttachment:
objCDO.AddAttachment Server.MapPath("reffiles") & "\myfile.gif"
objCDO.AddAttachment "c:\myfolder\myfile.gif"
But some limited testing showed this to work too:
objCDO.AddAttachment "http://www.thesite.com/myfile.gif"
objCDO.AddAttachment "http://www.someothersite.com/myfile.gif"