I want to add an alternative link to one of the URLs on the generated sitemap.xml by SXA is there away to do that manually? I have referred to that question but I think there may be a simpler way to do that rather than implementing my own SitemapGenerator
There are two things i would suggest here
Because i do not know for what purpose you want to have that extra URL, because as a best practice we should be only include the URLs which want the crawlers to crawl and which are for end user to see, and if you think your URL is such then i am just thinking why it is not in your content editor as an item so that sitemap will automatically generate it.
If point-1 is invalid and you think that URL should be there even if it is not there, you will need to have your own SIteMapGenerator patched to achieve this where you can write your own logic in the overrides
Let me know if that make sense