If you are new to WordPress but you have used some other Content Management Systems (CMS) before, for instance Joomla or Drupal, most likely you have themes for these CMS and you would like to use them as WordPress themes.
The good news is that this can be done. The bad news is that usually when you convert a theme from another CMS to a WordPress theme the result is hardly what you want. Generally speaking, there are ways to convert a theme from another CMS to a WordPress theme. However, if you dream that this can be done automatically, this is not so.
Most CMS have their proprietary theme requirements and it is not possible to create a tool, which converts from one CMS to WordPress. Yes, in many cases you can export the database you use with your old CMS and use it with WordPress but WordPress themes are different from databases.
One of the ways to convert a theme from another CMS into a WordPress theme is to hire somebody to do it. There are Web developers and designers who specialize in such conversions. Still, even the best Web developer or designer might not always be able to convert the theme from another CMS to a WordPress theme without substantial modifications.
In many cases it is best to start the development of a WordPress theme from scratch. Of course, you can use the images, colors, fonts, styles, etc. from your old theme but this is all you can borrow. Sometimes it is more difficult to adapt a theme from another CMS to a WordPress theme than to start from the very beginning.
Different CMS have different theme formats and it is quite possible (in fact, it's a fact) that your old theme uses items, which are not allowed in a WordPress theme. Well, if the old theme is really valuable for you, you might insist to have it adapted as a WordPress theme but all equal, when you migrate your site to WordPress, it is best to get a theme, which is specifically designed as a WordPress theme.
The advantage is that such themes make better use of WordPress as a CMS – in a sense, you get a piece of clothing made especially for you, rather than modify your older brother's/sister's clothes.
i agree with you, original themes are better than clones.