Welcome to our forum.
Polyamory is often difficult, but it can be done if everyone involved is willing, and it's usually worth it. The first order of business is to establish 100% honest communication, which, among other things, means that your relationship with "K" can't be kept a secret from your respective partners. You need an exit strategy from the situation as it is. Tell your husband that you have growing feelings for "K," and would like to be in a polyamorous relationship someday, if your husband would be willing. Tell "K" to do likewise with his partner. Do research about poly and keep the conversation going. In the meantime, if you can backpedal in your relationship with "K," do so. Refrain from sex with "K" for now, slow down on the romance in general, ease off on the IM's. If/when you and "K" have a green light from your respective partners, then warm it back up again.
This may not be an easy road to travel, but it's a necessary road to travel if you want this to work in the long run. Polyamory has to be done with the knowledge and consent of all involved. If it isn't, there will be disaster, drama, and heartbreak on the road ahead. You don't want that. Nip it in the bud now by changing your policy to a 100% honesty policy with your husband. Break the news gently, respectfully, but break the news.
I am happy to answer further questions or concerns if you post them here (or direct me to another thread if you start one). I sympathize with the difficult position that you're in, and I know I am placing a tall order in the advice I have given. Sometimes things have to be difficult for awhile before they can get easier.
Glad you could join us, in any case.