Yes, for joining LAN Argentina the applicant should have Argentine Passport.
Besides, LAN Argentina has not taken any additional aircraft since last December and there are enough FO qualified for upgrade for at least three additional aircraft so I don't see the need for DEC.
Every new joiner at DSM will be at the bottom of the seniority list (227 pilots at this moment) not depending in your previous experience.
If the company needs more Captains, they would publish the requirement and depending on the seniority list and the minimum requirements (4.000 hrs TT and 1.000 hrs in jets heavier than 55 tons) DSM will grant the slots for the upgrade.