I guess it is just a matter of proposing it to her then. Make sure you come to the talk prepared to answer lots of questions:
- Is she allowed to have emotional intimacy with others as well as physical?
- What are the rules for safer sex?
- Are you going to want the same kind of freedom from her?
- And if she isn't willing to give it, do you still want her to have her freedom?
- What are your insecurities approaching this (i.e. what could make you jealous)?
- What are your needs, wants, and limits?
The reason why one identifies as polyamorous need only be that they feel they can love more than one person. The reasons to actually open usually require a strong core relationship and consent and agreement from both partners that they can both be comfortable with that. It sounds like you have a great thing going with your wife, and I'm very interested to see how it turns out. :-)