Partner Contact User Creation not checking for existing user account

Summary:

When creating a partner contact user, if a user account with same username already exists, it errors out instead of using the existing user account

Content (required):

We have our CX Sales app integrated with IDCS with user provisioning. The user gets provisioned in the IDCS and a User Account is created in the CX. We then create a Partner Contact User to enable our partner users to use application, at which point, the User Account creation fails saying "A user with same username already exists in the identity store". We are looking for a way to do an Upsert on partner contact user creation. Any help is highly appreciated.