That's indeed what most games do with their DLC. I'm taking a different approach because the main game's visuals are outdated, and I want to revamp the prologue. However, I’m planning to release a patch where the new sprites replace the old ones in the main game.
Furthermore, while you can't romance Zeikun in the DLC, he does appear as a side character. I'm also planning to add extra stories that continue from the main routes, So you can enjoy Zeikun's story after the romantic ending. I hope this clarifies your question!