[Trisquel-users] How to create a "ffalse lenght" for a video file?

[gnuser]

Hey guys.
I want to create an empty file (preferably in MKV format) that has a false  
header so that if a media player opens it will think this  is a 3 hour long  
file.
Is it possible??
I will have mplayer dumping content into the file but I need the time to  
already be estabilished.

Thanks