Can I xxport a list of mp3s with length formatted in hours, minutes and seconds like hh:mm:ss?
Which variable have you used so far to get the length? Doesn't %_length% work?
See the documentation about available placeholders:
There are a lot of ready-to-use templates here:
and more specificly here:
You still you have to adjust it to your exact needs - or ask for it here with your needed end result example.