I know how to do it manually, but is there any tool to help me with that?
Here is a free and good step by step tutorial with video.
How To Convert an XHTML Website Template into a WordPress Theme
There are many tools on market which claims to PSD/Xhtml 2 Wordpress
but coding manually is the best way. No quick way to get custom and great themes.
This task could vary from rather simple to extremely difficult depending on the site you wish to convert over. Unfortunately there isn't a tool that will do the work for you. This task involves studying the WordPress codex in detail and inquiring as much as possible with other experienced developers.
It's a rather intimidating task, but the more time you spend in the documentation, the less intimidating the giant appears. I've been running myself over the coals for about a month now, and am working on my third conversion this week. The first two were terrifying, but they've been getting easier and easier.
Roll up your sleeves, and get ready for some serious studying!
The best and fastest way is by using http://wpsiteconvertor.com/, it's fast and you can convert unlimited websites which are all practically WordPress theme depository compatible. It creates a very proper WordPress theme in like a minute.