That depends on how Home Assistant handles SSL certificates. It seems to be fairly easy:
http:
ssl_certificate: /home/your_user/.homeassistant/certificate.pem
ssl_key: /home/your_user/.homeassistant/privkey.pem
That's just from a sample on their website (it's a topic about self signed certificates, but if you copy the cert from Ubuntu, that little part is enough), you'd need to change the directory to where your .homeassistant actually is.
Also, you could use Ubuntu as a reverse proxy. That's a much better solution, as you can use certbot or any other ACME client on your Ubuntu to automatically get the certificate et cetera. Way less hassle.