From the research I did, I dont think possible to add a user since the Users view displays users who's credentials are cached (users who have connected to K2). This just frees up licenses for other users.
You can read more on this via the link below:
I also check the DB to see if I couldn't add a user from there and unfortunately I can't add since they need LicensesUserID and CredentialID which are of data type uniqueidentifier. You can have a look on the attachment 20200817.
/****** Script for SelectTopNRows command from SSMS ******/
SELECT TOP 1000 [LicensedUserID]