I googled, but failed to find automated solutions. So, I've started it manually. Mapping CSS files are easy, but I'm looking for a way to map blocks/templates. Please, help.
There is no direct way to do it nor any programmatic way. This is because the template structures differ so much. But here are the steps you should take for the fastest conversions...
Its a lot of steps, but it works. You can also use a theme framework that provides a skeleton theme to build upon. The only problem with this is that sometimes these frameworks are very heavy handed and do far more than is needed. Zen is a common one, but it is very heavy handed. You can find more on the Drupal website at drupal.org (search for themes). Below are some links to a few base themes.
Finally, here is a link to Drupal's theming guide.
Sorry I don't have any magic bullets to offer you on this. I wish there were some.
I agree with pthurmond's answer. But I would suggest not to copy the whole html and css but to do it step by step. Copy the html first for, say, the menu. Then copy the css for it. Use firebug for the process. This way you will end up with only the really required things. Also I recommend a grid system line 960.gs as it is very flexible and you won't have any layout problems.