Can you do this? If so, how?
Do you just add it or will the fruit go bad?

THANKS!

Yes you can do this BUT fruit will only stay fresh for about 24 hours. Probably you can find a list in your supermarket that indicates how long you can keep it, but most of it (strawberries and raspberries) stay good for about 24 hours. On the other hand if you are using a mousse with whipping cream the cake has to be eaten rather quick too, so in your case I don't think it is a problem
And with a mousse and fresh fruit you have to keep the cake in the fridge and the right temperature.

What I often do is using mandarins from a can, make sure that they let them dry on kitchen towel and then put it on top of the filling, pusing it is pit into the filling so your cake is more stable. But as a filling I used most of the time SMBC and with the mandarins it stays well for a couple of days.

Good luck!! Y bon appetit!