If i have an HMO and i buy a supplemental insurance plan will the HMO refuse to pay?

no, they will be your primary insurance and the supplement your secondary. Depending on the supplement you pick up, it will cover what your primary (HMO) does not. Again this depends on the contracted coverage you have with the supplement. However, due to the contracted coverage with your HMO they cannot refuse to pay. Now, if you are single and the primary member for both insurances this is how it works. If you are married and your spouse is the primary on one of these insurances, neither can refuse to pay, but it makes a difference on which insurance is primary for you and depending on which ins is primary for you can depend in how they pay and what you will owe when they both have paid their portions.