<p>It's big problem to convert mod_rewrite rules to lighttpd format</p>

It is generally a case of just going through one by one and converting them.  I don't know of any automated means,
The docs -
http://redmine.lighttpd.net/projects/1/wiki/Docs:ModRewrite
- has the regexes available, and some examples.
if there are any particularly problematical items, I'd edit the question to show them, and ask for the answers here.