Robots.txt: virtual or actual?

Good morning,

I just created my sitemap using Google XML Sitemaps.
In default settings, the option “Add sitemap URL to the virtual robots.txt file” is checked.
Further, there is a note, ‘A real robots.txt file must NOT exist in the site directory!’
My SEO specialists request that I have additional content in robots.txt, such as

User-agent: Googlebot

If I uncheck the option and add a physical robots.txt file to my site directory with that content, could this cause a conflict with Google XML Sitemaps?

Thanks in advance

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