how to set existing site (containing sharepoint list) as a subsite of main site

[dogbural]

Hi, 

I have one existing site, having a sharepoint list listing data. 

Now I created a sharepoint communication site (main site), I would like to make the exiting site as a subsite so that when i search, this existing site can be searchable. 

Would you please advise me of how to do it?

Regards,

MK

[#2]

Why not define one of the sites as Hub Site and join the other one to the Hub? In this way you will have a search scope that allows you to search content in the entire hub