I don't know if there's a way to change built-in fade-in fuction properties.
I'm working on a project and I did this way:
I created a black object with the same dimension of the sceen and made it not visible.
On room transition, entering the room (on room-load) I make it visible, then (on room entering after fade-in) I set its transparency to 0; after that I change transparency to 100 (with the speed I want for the fade-in transition) and finally make it not visible again. When the player leave the room I set transparency to 100, make it visible, change transparency to 0 (with a certain speed) and ... change the room.
Maybe it's not a proper way, but for me it works well.
I'm working on a project and I did this way:
I created a black object with the same dimension of the sceen and made it not visible.
On room transition, entering the room (on room-load) I make it visible, then (on room entering after fade-in) I set its transparency to 0; after that I change transparency to 100 (with the speed I want for the fade-in transition) and finally make it not visible again. When the player leave the room I set transparency to 100, make it visible, change transparency to 0 (with a certain speed) and ... change the room.
Maybe it's not a proper way, but for me it works well.