Good morning ! Horatio and Riftborn are allies now and they asked me if I wanted to join their alliance. I refused because the game would end due to the conquest condition and I wanted to learn other mechanics (I also set all the conditions because this is my first campaign and I wanted to understand them). However, can I keep playing after the victory or loss message to do whatever I want ?     

Considering the wisdom behind your metaphor I may say this is an yes. Well, I'm going to settle a score with the Riftborn for using singularity on my system and with the Cravers who demanded 10k dust from my empire to "set me free".