You can add traits to users and companies manually
On the page of a single user you can find the plus icon that allows you to override the value of a given trait or to add a new one altogether
On company pages you can do the same:
Additionally you can add traits to users from the People list:
Lastly, if you're a little more technically inclined we have a CSV trait upload tool on Replit. This allows you to upload a CSV with traits for users in bulk.