Mix the raspberries and icing sugar.
Blend to a thick cream with a stick blender.
Imagine if all recipes where this easy.
There is frequently doubts about the health of frozen raspberries. As they are often the cause of food borne illnesses So unless you are certain of your source of raspberries you should heat it to 85°C (185°F). This renders it safe, but makes it a bit less fresh tasting.
It is not the freezing that causes problems, but rather unsanitary conditions during the picking, rinsing and storage of the berries.
So if you don't have your own raspberries, then it might be worth considering another dessert sauce :-S Or make it with strawberries instead.
You can make a coulis just like a jam, using the pound for pound of sugar and berries method. But I think it is better with more fruit and less sugar. "Pound for 1/2 pound" so to say. So if your bag of frozen berries weighs 300 grams (10 oz), uses just 150 grams (5 oz) icing sugar. There is no reason to have half a bag of frozen berries just lying around. On the other hand, if you want to stretch your berries a little, then just pour some more icing sugar in.
Any coulis that gets left over can be boiled and used as a marmalade.