create Sub-Domain for Domain with "Hosting Type: No Hosting"

Hey there,
is it somehow possible to create Sub-Domains for Domains with "Hosting Type: No Hosting"?
As soon as I assign "No Hosting" the Button for "Add Sub-Domain" is missing ...
I only need the Sub-Domain mail.dom.tld to generate LE-Cert for Mail-Services ... Web-Hosting is done on another Server ... would prefer not having a "Dummy No Hosting Service Package" with only 1MB Webspace + Hosting Type: Webhosting for this ...
thx, bye from Austria
Andreas