I very much doubt that this is literally possible. But what you could do is move files or folders from /dev/vda1 to /dev/sdb and then make them appear in the same place on /dev/vda1 afterward. This would have the effect of increasing the available space on /dev/vda1.
For example, a simple solution would be to move the contents of /var, /home, /opt, or /usr to /dev/sdb and then mount /dev/sdb as whichever folder you moved. That is a pretty common way to handle this. Be careful with /sbin and /root, as they may need to exist before /dev/sdb is mounted in your setup. You would do this by changing the fstab (which may be done via a tool in your setup).
You could also partition /dev/sdb (I presume). Then you could mount it as more than one directory. This would involve adding entries to the fstab.
A third option would be to move the files from /dev/vda1 to /dev/sdb and then symlink them such that they appear in the same place in the file structure. You would often put these symlinks at the folder level. E.g. something like
ln -s /mnt/volume_nyc3_01/usr /usr
You would do this as root (possibly with
sudo). This would allow you to put arbitrary folders or files on /dev/sdb while still accessing them at the same place in the file system. I presume you already know how to use
du to find which files and folders are taking up the most space.