I will copy paste my Support Chat as it sums it up best 
Andrew
Hi! This is Andrew from HubSpot Sales. I am here to help you better use our free products or explore paid products we offer.
03:55 PM
Hi Andrew. We plan to import our data into hubspot but we are currently missing a lot of custom properties. any chance we can import them somehow? or do we need to manually input them before importing our user data?
we were thinking alng the lines of creating a file like the export file you can generate for properties but then with our custom ones and importing this
03:56 PM
Andrew
That is a great question! I took a look into this and it appears this is not currently a capability of the HubSpot tools, and they would either need to be created manually, or this community post discusses a potential solution using an API endpoint for creating properties:
https://community.hubspot.com/t5/CRM/Importing-all-properties/m-p/403578
03:58 PM
yeah we found that one too but i thought maybe you know more
it looks like a lot of work to do it all manually as the mask for it is fairly extensive…
03:59 PM
Andrew
I totally understand that, I can certainly see how having a feature to import custom properties would be useful in many situations. I highly recommend posting this in our Ideas Board where our development team seeks out quality of life features to add to the platform!
04:00 PM
will do, thx
Done.
Let me know if this is ever invented please!
Cheers