Just add a script with the http://cs.elderscrolls.com/constwiki/index.php/PlayGroup function on loop:
SCN {Script Name}Begin GameModePlayGroup CreaturesDogIdleAnimsSpecialIdleSit 0End
If that bugs (as it might), you could simply just create a new Form, and then untick all of the animations in the animation tab, but the sitting animation that you want. (EDIT: It doesn't seem you can untick animations, actually, so ignore this line of text)
Hope that helped!