Broodmdh,
it all depends on what you mean when you mention "administration". If you are administering Windows Servers that work with AD on some networks that use AD features, then transition to a *NIX counterpart would mean
a) to administer a *NIX OS of your choice,
b) to administer
SAMBA. By that, I mean that there is the administration part of the operating system itself, and the administration part of the service you wish to offer. This is also true for Windows Servers, that have a part of knowing how to administer a service (like DNS, AD, DHCP) and how to administer the server itself (file permissions, tuning it, managing software, etc)
In general, typical *NIX administration usually involves file services (NFS, SAMBA, etc), web services (apache, thttp, etc), DNS services (ISC bind, etc), ftp services (proftpd, vsftpd), mail services (sendmail, postfix, spamassassin, clamav, etc), and others. Administering these services, means that you have to learn how they are configured (which is not always as easy as it sounds), how do they get started and stopped, and that's almost all of it. Sometimes, though, problems will arise that will be related to the OS underlying your services. When these problems arise, you'll have to be as well acquainted with the OS you've chosen as possible, in order to address them.
In order to move to administering a *NIX OS, you'll need to learn a few things with respect to *NIX architecture, *NIX commands, pick your OS of choice (eg FreeBSD), learn your OS' third party software management tool, and then its structure and idiosyncrasies (architecture, and supported drivers, filesystems, etc). Then, install the services you wish to offer and play with them.
Good luck