I don't think you can route through your cable box. You can get an HDMI splitter that will plug into one HDMI port but allow two cables to plg into it, I have used one and it worked well. You can also get an adapter for HDMI to another type of port but lose the HDMI HD.
Mis read your post - looking as if you had no spare port not no port! This may be what you need!! http://www.google.co.uk/products/catalog?hl=en&q=hdmi+converter&um=1&ie=UTF-8&ci d=12375205160434024531&sa=X&ei=NYJqUMajOeOB4gTXhoH4DQ&ved=0CD8Q8wIwAA