Login to Google Search Console. If you are in Newer version, switch to older version.

Go to Search Appearance > HTML Improvements

Click on Download this table button

Select and Copy (ctrl + c) the Links column from the CSV sheet

Go to your admin page > SEO Extensions > XML Sitemap Generator. And paste (ctrl + v) it into the filed as show below. And click Import button

You can see the links added to custom links tab

You can now resubmit your sitemap.