What's the best way to convert a Notepad++ style into a Sublime Text/Textmate syntax?
I've never found such a converter myself, but it wouldn't be too difficult to code up such a thing if you have moderate programming skills and spare time. Beyond that though, I think you'd have to do it manually, which actually wouldn't be too terrible.
Create a tmTheme file like this one: https://github.com/chriskempson/tomorrow-theme/blob/master/textmate/Tomorrow-Night-Blue.tmTheme
...and using this NPP theme as an example: http://waher.net/blog/Waher-style.xml
...just copy the fgColor and bgColor hex codes from the NPP XML file into the appropriate tags in the tmTheme file (e.g. String, Constant, etc)
Obviously there's not a 1:1 mapping of elements getting syntax-highlighted between the two editors, but with a bit of tweaking, you can approximate the NPP themes in Sublime Text.