The three add-ons (
The Stone Prisoner,
Warden's Keep and
Return to Ostagar, plus, if you bought them, the
Feastday Gifts), like their name say, can be played whenever you like during the Main Campaign, as long as you're not in the final stretch of the game.
As for the Other Campaign, there's really no intended order, aside from the realising one. If you want my advice, you can perfectly skip
Darkspawn Chronicles, which is quite dull in my opinion, while
Leliana's Song is extremely good, with a nice story, a nice narrative and some interesting quests. I'd suggest you leave
Witch Hunt for last, since it's the perfect DLC to wrap up the Warden's story, at least for now. Finally,
Golems of Amgarrak has plenty of atmosphere, a nice little plot and stirking visuals but the DLC stands out for its insane difficulty (just for example: there's a battle that, at Normal (!), I had to do fifteen times before I was able to beat the monsters, with only my Warden staying alive through the battle... And no one ever died in my party throughout the entire Original Campaign, not even when I played at Hard!)
The biggest question, though, would be be when to play
Awakening. Well, I'm pretty happy with how I organized the different campaigns and I think that this arrangement makes the most out of them:
Dragon Age: Origins (Original Campaign + Add-Ons) > Import your Save in
Dragon Age: Origins - Awakening > Import your Save in
Golems of Amgarrak (optional) > Import you Save in
Witch's Hunt
The other DLCs can be played whenever you like
