Domain mapping map customer's sub-domain

Hi there, I was wondering if Domain Mapping could help me here:
I've WPMultisite with the following:
> sub site1:
> substite2:

And I'd like:
> subsite1:
> subsite2:

For the 1st one, no problem with domain mapping, but what about the sub site 2 with customer's subdomain mapping? Is it possible? I've read something about CNAME config but I am really not comfortable with all of that.

Thanks for advice,