Question on Mariners Records held in Archives

My grandfather had the surname Sweetensen. He began living in Cardiff, UK, in 1903. According to my father, he was born in Riga Latvia(then Russia) in 1886 approximately; however, in some records he has listed his origins as Kristiania. We think perhaps as a teenager he travelled to Kristiania from Riga and made it his base until 1903. At some stage he would have changed his name to Sweetensen from his original name (something like Kushkin). We are wondering whether, if not native born, his details would have still been recorded in Kristiania for the purposes of national service.