It is not possible to export a droplet, unfortunately. Please vote for this feature on our suggestions board: Download snapshot and/or backup – Customer Feedback for DigitalOcean.

If you need to export the droplet, you can migrate it manually, see How To Migrate to a New Linux Server.

Actually, yes you can!
Excellent write up of the process using VMware ESXi server. Free temp license works fine. After you complete the process it is easy to export the machine to VBox.

https://www.joe0.com/2017/01/12/how-to-convert-a-remote-physical-linux-server-to-a-virtual-machine-vmware-virtualbox-instructions/

For DO, you will need to temporarily enable ssh root access on port 22, not ideal.
After downloading the server you may need to reinstall the kernel, depending on your server. You should still see grub during boot up, indicating a successful clone, but it may stop there.

You will need the standard boot-repair iso:
https://help.ubuntu.com/community/Boot-Repair

Reinstall kernel by using the boot-repair disc
https://askubuntu.com/questions/28099/how-to-restore-a-system-after-accidentally-removing-all-kernels

Before you reboot the system disable module i2c-piix4
Otherwise you will get an error like this:

Piix4_SMBus: 000:00:07.3: Host SMBus controller bus not enabled

Edit the file here:

nano /etc/modprobe.d/blacklist.conf

go down a few lines (almost anywhere) and enter the line

blacklist i2c-piix4

To enable networking, assuming you have a static ip set.

nano /etc/network/interfaces

change the line starting with iface to this

iface etho inet dhcp

Comment everything out after that . . .
Restart the server.