can i add existing sites to multisite network?

I have a main website and two subdomains. Each subdomain has a different demographic, so they have different themes, etc.

I am in the process of transitioning to MultiSite because it will give us more of what we are looking for.

The instructions tend to focus on adding subdomains (or subdirectories) at the “beginning” of building your site / blog.

We are past that and have already ID’d our structure and have the sites working.

Can I move them into the network without having to start over?

Thanks in advance! Terry