Unfortunately when it comes to our feelings we have little control as far as who our hearts pick to have an infatuation with. Having said that, I believe the Great and Mystical Dan Aykroyd best summed it up in the Movie Dragnet….
“Now let me tell you something, Streebeck. There are two things that clearly differentiate the human species from animals. One, we use cutlery. Two, we're capable of controlling our sexual urges. Now, you might be an exception, but don't drag me down into your private Hell.”
While you may not have control over whom your heart picks, this does NOT mean you have to act on it.
First off, More than likely they are NOT in an open relationship and that you have no possibility of a relationship with her and secondly even approaching this subject with her could not only jeopardize your position with the company (as I assume this could be construed as Sexual Harassment) but will most likely also make things uncomfortable at work especially if she has no interest in cheating or a Poly/Swinger relationship.
My Advice… Don’t do it, and go find someone who is actually available and not in a relationship. Sorry to be the bearer of bad news, but I just don’t see anything good coming from perusing this relationship.