That is currently not possible, you can allow them to only create subchannels while they are in a certain channel, which effectively makes it so that all channels they create are subchannels of that particular channel. But you cannot allow them to create channels anywhere and still have them all appear as subchannel of one particular channel automatically.
Other choice would be to let them create channels but not allow the sort order to be changed, which makes the channels end up at the bottom.