Chriatian,You have at least three options, if not more, depending on how picky you are about the final output. The easiest is to write the music as usual, then go to the options menu > document settings > barlines and uncheck all of the boxes. Then use the staff attributes dialog to hide the time signature. You may have to tweak the numbers in the Music Options dialog a bit also.
A second method involves creating a time signature that accounts for the total value of the piece, i.e., 125/4 or something like that. You then enter the notes for the whole piece in a single measure, hide the barlines and time signature and you are set. Problems will arise when you get to the page layout if the piece is very long, though.
A third option is to set up several measures of varying lengths, enter the notes, then hide the barlines and meter signatures. This method takes a little more planning, but will probably give you a better looking printout.
Good luck.
Jim Coull