Download the official extention for inkscape
Esoteric sofware has official extentions for the most of the graphics editors. The extention for inkscape you can find here.
For our case we need only two files objects_to_spine.inx and objects_to_spine.py
Save the extetion to the extention's folder
In OSX the extention's folder placed here '/Users/you-username/Library/Application Support/org.inkscape.Inkscape/config/inkscape/extensions'. So just add your folder and past the files.
Use Export in Inkscape
In Inkscape just select objects that you want to select and open the extention. 
Import the JSON file into you spine project.
There is pretty standart and usual action. Just import it. 
That's all! I hope this guide will help you. You can see the full setup in the demo project.