Astaro uses FreeS/WAN, so you should be able to follow their instructions for setting up between two FreeS/WAN units, then "translating" into Astaro. If you need help with the "translation," please post your questions inn this thread.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005