Using an SD card for extra space on Bootcamp

I finally got Bootcamp working on my Mac so I could play Windows Steam games on it. The only problem is that I don’t have enough storage on Bootcamp to download games from Steam and I ran out of space on my Mac to put more space on Bootcamp. I bought a 64 GB SD card to fix this, but I can’t figure out how to actually store the SD card storage on my Mac. I converted the SD card to an external drive but that still didn’t solve it because I can only partition the SD card and I can’t add it to my Mac’s storage. I also can’t add the external drive to my Bootcamp storage, and whenever I try to use it on Bootcamp (like installing Steam on it or installing Steam games), it won’t work. Is there any way to just add the external drive to my main Mac storage or to the Bootcamp storage?