There is no confirmation that said DLC even takes place during the Epilogue.
Consider that Lords of Shadow during the game itself takes place in the year 1047 or something. For all we know said DLC could take place PRIOR to that Epilogue, building up events slowly to reach that point.
Nearly 900 years is a very, very long time for be explored.
Take example of GTA4 Episodic DLC while based on the same game engine are essentially self contained stories with new characters that expand on the existing universe.
Castlevania : Lords of Shadow could as well also have Episodes filled with original content and new story segments that could fill in the gaps for those period of 900 years or so.
For example the game could go into events just after the final battle
when Gabriel starts to slowly transform and eventually become Dracula