^He's a member of SG-1. SG-1 have snuck on to ships a lot more heavily guarded than an old freighter in their time. He has USAF Special Forces training roughly 75 years ahead of anyone else's at the time too.
Good thought,but no.
Cam Mitchell was a crewmember aboard Achilles,and there's several reasons why.One,Mitchell had 10 years to prepare.Its reasonable to assume he could have become a seasoned mariner in that time,enough so that he could be hired by his grandfather.And claiming family relation couldn't hurt at the job interview.
The second evidence is found on-screen whence Mitchell orders the crewman to take a weapon ,as he explains offhand they're being boarded.
Now,if a stowaway stuck a gun in my hand,said we were being boarded,and with the cargo we took aboard making noise, I'd point the stowaways own gun back at him and start asking ?'s.
But no,the crewman takes the Thompson and stands post in front of what appears to be an average box ,to his understanding.Without question or hesitation.
After that scene,the captain comes down with his revolver to see several dead Jaffa and a hole in his ship.If Cam was a complete stranger the captain would at this point *also* be aiming at Cam,with some very good questions on his mind.But no,the man in charge puts down the pistol and asks Cam what happened-as he would ask someone he knows,not an armed stowaway.