convert from single site to multisite (network) with existing subdomains in WordPress

This seems possible to me. Most of this is related to server management andt only WordPress, so probably Server Fault also has interesting info on this .

On the DNS side, you would create a wildcard to the server with WordPress, and define your other subdomains to the servers that host those (some or all of them can be the same as the WordPress server). Wildcards always have the least priority .

For the server side it is useful to know that if a request reaches Apache but it finds matching ServerName or ServerAlias in a , the first virtual host is returned. So you configure the WordPress setup as the first host and the others running on that server after it.