One viewer needs an extra heat grill in the kitchen, which is right above the furnace room, so running ducts is not a problem. The question is, how do you attach to the existing ductwork?
You do it with what we call a "boot".
You will have to cut a square hole into the large heating duct and the fastest cleanest way to do that is with a jig saw with a fine metal cutting blade. This task is harder on your ears than anything else. The boot will have a slope on one side. Think of the air flow and place it so that the air can slide most easily along that slope as it flows into the boot. If you put it backwards, it will slow down the air.
Simply pop the boot into the hole and then reach in and bend the little tabs to lock it into place. Don't forget to go back and tape the joint air tight with aluminium duct tape. Don't use cloth duct tape, it will fall apart in short order from the heat.
Now you can put on regular round ducting and the floor boot and the grill. Remember that the heating system is a whole systems with only one fan, so when you add a new duct like this close to the furnace, you will have less air throughout the rest of the house. You may have to play with the furnace dampers, perhaps even install a damper in this short line, to re-balance the whole house making sure that the remote rooms still get a good air flow.