User @wachidadinugroho has worked out the details on how to setup a shared folder to be used between Waydroid and your host filesystem.
Setting up a shared folder will allow the user to copy/paste files from the host and they appear inside waydroid/android.
sudo mount --bind <source> ~/.local/share/waydroid/data/media/0/<target>
Then verify that the target folder exists:
sudo ls ~/.local/share/waydroid/data/media/0/
Examples:
sudo mount --bind ~/Documents ~/.local/share/waydroid/data/media/0/Documents
sudo mount --bind ~/Downloads ~/.local/share/waydroid/data/media/0/Download
sudo mount --bind ~/Music ~/.local/share/waydroid/data/media/0/Music
sudo mount --bind ~/Pictures ~/.local/share/waydroid/data/media/0/Pictures
sudo mount --bind ~/Videos ~/.local/share/waydroid/data/media/0/Movies
You can also make your own custom mount point to cater to your needs.