The 38-year-old, whose last game was for his old club Helsingborgs against Ipswich Town in the UEFA Cup on November 1, says: "I will do my best and do the job, but West Ham have David James and Shaka Hislop so I know I am just back up for them - but you never know.
"My family will come over if I do play - there is a big interest in English football in Sweden."
Sven was due to give up playing and take up a coaching post early in the new year, and says: "I should have been retired so it will be very strange but it will be fun to play for another six months.
"It will be really interesting for me to join a club like West Ham - going abroad was a big decision but I am happy."
He admits he does not know how Glenn Roeder came to discover him, but he surmises: "Maybe they heard of me because I played for Helsingborgs against Ipswich in the UEFA Cup.
"There are better players in England and everything is much bigger, but my Swedish club have played some good teams in Europe in the last six years."
He admits the move might even reawaken his appetite for the game and extend his career beyond the summer and he adds: "You never know; you should never say 'no' but we will see what happens."
MD Paul Aldridge adds: "We are delighted to have secured Sven's services until the end of the season and the players are looking forward to working with him."
There is no fee involved.