Well, a good place to start, if you haven't already, would be to talk with your spouse.
However I will warn you of something - I don't know what you feel is "missing", but one of the things folks advise strongly against is the idea of "relationship broken, add more people" - it never works.
Could you talk more about your situation and what you have discussed with your spouse?
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb