import .css file into .less file

Change the file extension of your css file to .less. You don't need to write any LESS in it; all CSS is valid LESS (except of the MS stuff that you have to escape, but that's another issue.)

Per Fractalf's answer this is fixed in v1.4.0

I had to use the following with version 1.7.4

@import (less) "foo.css"

I know the accepted answer is @import (css) "foo.css" but it didn't work. If you want to reuse your css class in your new less file, you need to use (less) and not (css).

Check the documentation.

If you want your CSS to be copied into the output without being processed, you can use the (inline) directive. e.g.,

@import (inline) '../timepicker/jquery.ui.timepicker.css';

You can force a file to be interpreted as a particular type by specifying an option, e.g.:

@import (css) "lib";

will output

@import "lib";

and

@import (less) "lib.css";

will import the lib.css file and treat it as less. If you specify a file is less and do not include an extension, none will be added.