I'd like to implement this feature, too, but unfortunately, the BootsFaces datatable is based on the JavaScript datatable hosted at DataTables.net. Actually, that's a really great table widget. BootsFaces has just begun to expose the host of features of dataTables.net to JSF programmers. So we're not going to replace it with something else anytime soon. But dataTables.net does not offer expandable rows or tree tables.
That doesn't mean it's impossible to implement such a feature. There are several third-party libraries adding tree functionality to the datatable. One of the more interesting (at least at first glance) libraries is https://github.com/shingos/datatables-treetable. We may or may not use this extension in order to implement expandable rows in particular and tree-like rows in general.
By the way, if you want to speed up the process, and if you're familiar with JavaScript, feel free to send us an issue report or even a pull request at https://github.com/TheCoder4eu/BootsFaces-OSP.