In the Construction kit I saw that in the dragons' spell list there is an unrelenting force shout, but they never use it in game. Is there a way to change this and maybe to add other shouts as the dragon version of storm call and disarm to their usable shouts?

If you alter the dragon spell list, you can add it in there. I have no idea if it works, but just customize the spell list and you should have dragons that do that.

However, I'd make a custom dragon to test it first before tinkering with the main dragon race.