I found my adult rabbit as well. I don't know her exact age, but vet said likely between 2-4, but likely guessed lower age range of the spectrum. Recently finished litter training her, and it was rather fast and easy the method i used. As Jbun said, find out what corner they pee/poop in, or if it's multiple, add another litter box, it's what I had to do. I used different materials in each eventually causing my bun to pick the spot she preferred and allowed me to make a "chill/dig" box when it's too hot inside or when my bun wants to dig. While also blocking off the other spot not allowing her to pee/poop there. It looked like this: One had a hay feeder with no hay on the litter(other then what fell from the feeder) with a different type of litter while the other had my "recipe" of Aspen/soft wood mix with hay on top and no feeder. She choose the 2nd choice and stuck with it, and was trained within days, and mine is an indoor/outdoor mix, but even when i caught her, she would spend around 10 or so hours outside in her outdoor playpen then come inside for the night.
What I turned the second litterbox into after she choose the one she liked better, even when i extended my indoor playpen, she knew where to go. i just need to make the dig box out of wood now lol. I still leave it there as this was a corner she liked to pee/poop in and in leaving something there, keeps her from trying anything in this section. I probably could remove it entirely if I wanted to just to see, but why mess something up that works haha.
What she is normally used to(without all her holes she dug, older picture it's the first day i set it up), so yes it's 100 percent possible to litter train them even if they were outdoor buns. If you need any help, feel free to PM me or ill keep answering here so others in the future can see what may help as I have experience with outdoor buns becoming indoor buns as California summer means my bun well become a full time indoor bun for 3-4 months and why I had to end up litter training mine. haha.