It is possible, but not ideal. Your best bet is to crack open your terminal and run
sass-convert --from css --to scss
in the directory where your styles are located and then refactor from there. It's a pain, and not a true "conversion". It tries to nest properly, and usually gets it right, but if you're not refactoring the code and using things like mix-ins, extends, and variables, it kind of defeats the purpose of using SASS to begin with.
To automate this, you may want to use something like Guard to watch the css files for changes. You'll want to make sure you're not also watching your main style (the one that sass is compiling to), as this will put you in a never-ending loop of conversion!