Yes, they could! That's the fun part! Including the one where Thor becomes a member of SG-1 while Carter goes off to study on the Asgard homeworld. Don't worry, it only lasts a season.
Why do you think the show got canceled? Thor was a horrible replacement, the audience just didn't connect. Then Sam gets shipped off to Atlantis to have Lemonhead's baby.
And Teal'c was already replaced with evil Teal'c. Where do you think the hair came from...