You can post to that model.
You will want to filter the feed item list by “parent ID” - which is a polymorphic field allowing about 90 object types. You will only want to allow selection of the “Group” object. Here is a post with some code showing you how to do that:
https://community.skuid.com/t/can-i-limit-which-objects-are-possible-targets-of-polymo…
Let us know how things go? Maybe post some XML as an example if you feel it is successful!
