Hi Pilou - you may want to use Draw curve > Freeform > Control points for something like that, snap each control point on to a polygon vertex and the result should be a shrunken down curve contained inside the polygon.

Right now there is not a good way to script that, because the endpoints of individual segments inside of a polygon curve are not accessible by script, that's something that I'll need to add in for v3 in order to make it possible to script it.

- Michael