WPF: DataGrid with a tree-column in an MVVM Windows application

WPF: DataGrid with a tree-column in an MVVM Windows application

I'm working in an MVVM environment for a windows application. There is
already features do add and remove column-types (string-column,
number-column) in a DataGrid, reload data from the database when a column
gets visible that needs it etc...
Code is quite big and divided so excuse me not to post it. This also means
that there is no room for a 3rd party solution because it should work with
the existing/own framework
Now I have to add a new column-type "treeview". It shall look like the
treeview control and also load data of the children only when the user
opens a node. So the levels are not defined.
I found a lot of solutions but none of them fit for me as at least one
requirement (datagrid, mvvm, wpf, ...) is not met.
Thanks a lot Soko