Here's how I converted a whole batch of Stylus files to SASS without using a converter (none exist, that I'm aware of), converting each Stylus file manually, or programming my own Stylus->SASS converter, which would have entailed building a parser, and then generate SCSS from the AST.
First, grab sandr.py here: https://github.com/jfgiraud/sandr
Then, in the directory of your Stylus files, run (if you have multiple directory levels, you can do similar task using find):
for file in *.styl; do echo "/*! FILENAME: $file */" >tempfile; cat $file >>tempfile; mv tempfile $file; done