At the moment, I'm afraid it isn't possible to customize meeting properties such as meeting outcomes. In terms of customization, there are custom call and meeting types if you have as Sales Professional Subscription and above.
I did some research and was able to find this post in the HubSpot Community Ideas forum. I would urge you to upvote - our Product team regularly reviews new ideas, and many of our newly implemented features have come from this forum - this would be the best way to let them know that this is a feature you're interested in. Doing so will also auto-notify you if/when this functionality becomes available within HubSpot. You may read more about how that review process works here.
Re Meeting Properties - Outcome Options.... Can you add to standard subscription the meeting outcome option "tentative" or recommend how to note that you've reach out to set a meeting but it's not yet fully confirmed / scheduled. I would like to still see my "tentative" meetings on my report dashboard.
in order to create custom calls and meeting outcomes. If in the future we have the option in other Hubs, the page I just sent will have the updated information.