The Bundesliga club said the 31-year-old Marlet had signed a one-year contract with an option for another season, while Alex had joined on a four-year deal.
Marlet joined on a free transfer while media reported Wolfsburg paid around 1.8 million euros ($2.20 million) for Alex.
'Steve Marlet is very well known in international football , a player who is fast and versatile,' said Wolfsburg manager Thomas Strunz at the presentation of the two players on Sunday.
'Alex is less well known, despite being an international, but he is technically very good, like all Portuguese players, and has exactly the right qualities for a right-sided defender.'
Marlet moved to Fulham from Olympique Lyon in August 2001 for 18 million euros. He returned to France on a loan deal with Olympique Marseille in 2003.
Wolfsburg are ninth in the Bundesliga after drawing their opening two games of the season.