Hi,
No. You can't exceed licencing capacity through ArcGIS Online administration.
However, you could temporarily remove one from your organisation and then reinvite them. Would work OK if they are a user type with no content or group ownership (I e. not a Creator).
- Take note of what groups the user is in
- (If applicable) transfer content/group ownership
- (Optional) enable ESRI access on the user account. This will allow the same user account to exist after removal from the org. Applicable if they have done E learning, access my ESRI, or post on forums etc
- Remove the user from the organisation by deleting the account
- Change the other user type
- Reinvite the user account (if ESRI access - account still exists) or recreate
Otherwise ESRI support can potentially assist with temporary additional licences but it's usually more for cases when there is only one creator for the entire org and an admin needs to be swapped out, etc
