Hello,
It's possible in theory, but not via WebUI. You can upload a key via SSH and then modify the OpenVPN server's config to point to that key.
Upload your key to the router, for example, to the /lib/uci/upload/ directory (where all other keys are uploaded by default).
Then you have to add this line to the OpenVPN config file:
option tls_auth '/lib/uci/upload/key_name.key 0' (replace the 'key_name.key' with the actual name of your key)
You can do this by editing the /etc/config/openvpn file with the vi command.
Instructions on how to upload files to the router can be seen here (for Linux) or here (for Windows).