Multisite Custom Domain Mapping htaccess


We are launching a multisite solution. Some of subsite will use their own custom domains.

So out domains will look like this:

In our solution anyone registers on any of the subsite will automatically get an account with the main site.

in this case, is it possible to let the user stay in the network. So that when a user goes from the main site to or vice versa, he will not be logged out.

Is it possible? if so, what should we do with the htaccess? ( has default htaccess for subdomain. We are not using a subdomain but custom domain.)

Thanks a lot.