The library support is a feature of KLayout. It generates a specially crafted GDS file structure to support this, since GDS itself does not.
Of course, with Nazca you can import cells from KLayout libraries, just as you could with every other GDS file. See for example the tutorial make a BB from GDS.
This will even update your GDS when the library changes, but you have to rerun the Nazca source program. Full Nazca support for the KLayout library feature is possible, but it is not there at the moment.