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.