Can I add a user only for Motivator without adding to CRM?

Can I add a user only for Motivator without adding to CRM?

No. All user needs to be a CRM user.
Once you add a user to CRM,  the user will be automatically synced with Motivator.

See also:
Add a CRM user
https://www.zoho.com/motivator/help/settings/user-management.html#Add_Users