According to an in-game update, this crossover will take place in Chapter 5 Season 3: Wrecked "courtesy of the Brotherhood of Steel", with the chapter arriving next week on 24th May 2024 (via IGN).
No other details have been revealed just yet, but as can be seen in the artwork below, expect power armor.
If you're looking for a Fallout fix on the Switch, you'll have to settle with the free-to-play title Fallout Shelter for now. As for Fortnite, it's free to download from the Switch eShop as well.
Epic also recently rolled out LEGO Fortnite, including a new crossover with the LEGO Star Wars line.