Sm2tap is a project mainly written in PYTHON and SHELL, it's free.
StepMania to Tap Studio Convertor
StepMania to Tap Studio convertor. (Requires a Mac and a jailbroken iDevice)
Please read my blog post for more info.
plutil -convert xml1 /path/to/your/song.tapd
python /path/to/convert.py /path/to/your/song.sm /path/to/your/song.tapd
plutil -convert binary1 /path/to/your/song.tapd
.tapd
file back.