AFAIK you can only domain map your blogs if you are using subdomains for the sites instead of directories.
However, it should be possible using mod-rewrite and a .htaccess file to rewrite https://www.domainB.com to https://www.domainA.com/siteB.
Doing it that way https://www.domainA.com/siteB will remain and continue to work.
A small side effect of using mod-rewrite is that this really only applies to the initial entry to the site because after that they would have to use the https://www.domainA.com/siteB format as that is the actual site url.