You're correct, the nunation suffix does not go on foreign names. Seth/Seeth, however, is an Arabic word. I did a little google research. Seth was the third son of Adam and Eve. Though he was not mentioned in the Koran, he is an "anbiyah prophet" and was/is spoken about and written about in Arabic literature.
I tried to look up the name in four Arabic names for boys lists but couldn't find it.
Also, the nunation should be "-un" here and not "-in". "-un" is nominative/subjective while "-in" is genitive/possessive.