Yes, you can use emoji in the subject line of your HubSpot email.
To do this, you can navigate to a free online emoji provider, copy the emoji you want, and paste it into your HubSpot email subject line.
Please note: Not all email clients support the use of emoji. For example, Gmail and Apple Mail support the use of emoji in an email subject line, but emoji will not display in Outlook 2003. You can take a look at Campaign Monitor's resource on all supported and non-supported email clients for emoji.