Question:
Preferably using gulp-sass or another gulp plugin
Answer:
The sass-convert
plugin can be used for conversion.
Usage example:
var vfs = require('vinyl-fs');
var converter = require('sass-convert');
vfs.src('./input/**/*.+(sass|scss|css)')
.pipe(converter({
from: 'sass',
to: 'scss',
}))
.pipe(vfs.dest('./output'));