Currently if a recurring meeting is already configured without a webex component a single instance cannot be modified to add a webex meeting. The wording for the message is "You must add the Webex meeting to the entire meeting series first before you can add it to this occurrence"
Are you referring to the Outlook Add-in behavior? If so, this is listed on our Issues and Limitations page "Adding a Webex meeting to a single occurrence in a recurring meeting series that was previously scheduled in Outlook is not supported." https://help.webex.com/en-us/article/n194fs/Known-issues-and-limitations-for-the-Webex-Scheduler-for-Microsoft-Outlook