I'm kind of unsure whether this could be accomplished with a common File Dialog or an underlying yellow File Dialog and its ExtFileDialog internal function. But you could switch to .NET and use some third party libraries available. One of those is BetterFolderBrowser proposed here. I have just tested it on both 32- and 64-bit LabVIEW 2019/2020 and it works great. Here's the basic example:
Untitled 1.vi