Is there a tool available for converting a manually created CSS file into a nested, well optimized LESS file?
No. Time to roll up your sleeves and re-create whatever css you have into .less files.
If you are building a framework, I highly suggest looking at this, as it contains many variables and mixins that are helpful for .less files.
Keep in mind that while {less} is awesome, you should not use it for deployment. I suggest using your .less files for development only, and compiling them into minified .css files for deployment/production.