Yes it is, move them all into an album on your N900 with drag and drop with a mouse when in Mass Storage mode. Then drag that folder into mp3tag and press <ctrl> + <a> to select all songs, and on the album tag, put in whatever you like.

I've done this before. I have a folder called "loose songs" for all those singles and songs that I have on there but not the whole album. The only annoing thing is that any picture (album cover) attached to the song will follow it around everywhere.