Jecht got to Spira that same way Tidus got to Spira, he touched Sin. He went out to see to I think train, and Sin was there, and he touched Sin, and Sin brought him back to Spira. Same with Tidus. I think that it was accidental with Jecht, but I think it was on purpose with Tidus. It might not have even been accidental with Jecht, who knows. And who knows if Jecht was the first person from the dream Zanarkand that this has happened to.
What was Sin doing there though? As far as I know, the fayth for that perticular Sin had no connection to Dream Zanarkand.