igmp-proxy destination nat

Hello experts,

Could somebody explain me how to solve my idea. I have an IPTV STB connected trough mikrotik. IGMP proxy works well and everything works fine. However STB does not show some extra channels which I can access on PC. There simply is no option to choose them. My idea is to set up a dst-nat and watch these streams which I want instead of some other. However I do not know how to set this up. Multicast packets seems to be going trough IGMP-PROXY and ignores any dst-nat rules. I can catch these packets in firewall or mark them with mangle but I ran out of ideas how to change destination address. Hope for some ideas. Many thanks in advance.