Not currently possible by drag+drop on sidebar*
Something i was toying with was showing a separate tree entries for sublfows templates that contains UI nodes - permitting user to drag-drop sort them. it was out of scope due to not knowing if the feature would be used or desired. Fortunately, this is all in the dashboard side. so once NR4 is "current", and the feature is fully released, an update to dashboard is possible at a later time.
BUT it is quite a large piece of work so not likely to drop any time soon
What MIGHT come before that ^ is exposing the sort number as a number entry field when a UI element is inside a subflow template (like we did for the sizer) - that would be much simpler to deliver and would start being useful sooner rather than later.
* If you really need to sort them, at the moment, you would have to cut them from the subflow, paste onto regular flow, sort in the sidebar, cut and re-paste/re-connect in the subflow. (or export, manually edit, import)
