After trying out so many ways i finally figured out how to add nokia music store downloaded mp3 to itunes.
Download Windows Phone connector app from appstore.
Import songs from phone to MAC (I worked with itunes on MAC)
Open song with Quick Time player
Go to 'Edit-->Trim'
Now adjust the trim space. Trim it just a little so that you dont miss even 1 second of song.
'File-->Export To' the trimmed song to another location
Go on and open in iTunes now.
i wanted to get some ringtones created out of downloaded songs. So i had to anyway trim songs.
I believe Quick Time player readjusts the mp3 header and makes it compatible for iTunes to understand it.
Any way, who cares about the technicality of it when it works fine
enjoy
nik