I don't have a definitive answer for you, but maybe a couple of things to think about. The acid in the fruit will lower the acid in your wort. How much will depend on how much fruit you add. But yeast gets grumpy if the pH drops below 4, and really grumpy under 3.
Depending on how much the Roselare blend has dropped the pH, you might need to rely on the Brett and bugs to ferment any sugars the raspberries add. I don't have any idea how long that would take.
If it were me, I would wait until the lambic is finished, then ferment the raspberries separately, maybe with a little fresh wort for nutrition, then blend the two to taste. I think you would get a fresher raspberry taste. It's also possible I have no idea what I'm talking about.