I think there is no easy way to get what you want but it should still possible to achieve this.
Via "Call web service" action, you're able to call "SharePoint Permissions" web service :
In order to get permissions for a group on site A.
In order to set permissions on site B. You will probably have to manipulate XML obtained by the first web service to give it to the second one.
Hope it helps
Hi Matias Ramirez,
If I were you, I would look into the tool 'Sharegate'. Yes, it is not free, but building your own is also expensive and this is much more generic so you could use it for other situations too. It really has powerful features.