CSharpUser 0 Light Poster

Hi,
I'm using taglib-sharp to modify MP3 files (e.g. assigning a new title) after I've "stitched" them together. I also want to modify the Duration property (mp3.Properties.Duration) but of course when I try I get the error that the property is read only.
I admit to being no expert in C#; so I am hoping for some help here... How can I modify the duration property of an MP3 file using tablib-sharp - or any other method?
Thanks for any help provided!