In my understanding, if you have text, it's not a challenge to train on next word prediction. Just keep the learning rate low. The reason there's a focus on the instruction based fine tuning is because that data is harder to come by.

My only experience is I've done this with a sentence embedding model (using sbert) and I just trained on my new text and the original training data 50/50 and it both got better at embedding my text and didn't forget how to do what it was originally trained on