A codepen or snippet would clear up what you're trying to do.
I understand that you're creating lines on a leaflet map from multiple features in a geojson using L.geojson and now you want to use an external library that requires an L.Polyline instance?
If so, I think you'll have to iterate over the features array in your geoJson and generate the lines. Then you can pass them to your library.
One other thing - Geojson coordinates are lon lat while you'll need lat lon for creating L.Polylines.