Incorrect. It's possible to create datablocks after the server starts, although they will not get downloaded to clients unless they rejoin, or the map is changed. If the datablock is spawned before they have downloaded the datablock, they will get a Invalid Packet error and disconnect.
Although it is possible to re-transmit datablocks without re-entering the loading GUI (load add-on, etc), it is not failsafe and sometimes does not work.