Adding custom properties to OOB sharepoint webparts

What is the best way to add new custom properties in WebPart Editor Pane of the OOB SharePoint WebParts (such as Content Editor WebPart & Script Editor WebPart which are created as sealed class)?

My main goal is to introduce new custom properties in the edit panel of the OOB web parts. I'll use SharePoint REST API to fetch the property values.

I've already come across the following articles but it looks like we can add custom properties to only Custom developed Web Parts. I want to utilize the existing capabilities of CEWP or Script Editor WebPart and introduce new custom properties (that can be used from within apps hosted in these OOB web parts).

<a href="https://www.codeproject.com/Articles/31990/Custom-Content-Editor-Web-Part-for-SharePoint?fid=1532859&df=90&mpp=25&prof=False&sort=Position&view=Normal&spc=Rela">https://www.codeproject.com/Articles/31990/Custom-Content-Editor-Web-Part-for-SharePoint?fid=1532859&df=90&mpp=25&prof=False&sort=Position&view=Normal&spc=Rela</a>xed

<a href="https://msdn.microsoft.com/en-us/library/office/hh228018(v=office.14).aspx">https://msdn.microsoft.com/en-us/library/office/hh228018(v=office.14).aspx</a>

I believe the only to achieve this is to use Client side web parts developed using the latest SharePoint Framework which only on SharePoint 2016 Feature Pack 2 and SharePoint Online. Unfortunately, I'm on SharePoint Server 2013 Standard and trying to figure out an alternate way to achieve this.