I needed to add multiple domains to search DNS with on my Cent7 box. It turns out there are two ways to do it. Cent7 uses networkmanager, so you can use the cli tool to add what you want, or you can edit the file directly.

Using nmcli:

sudo nmcli con mod eth0 ipv4.dns-search "domain1.org,domain2.org,domain3.org"

This causes nmcli to add this line to your network interface config file (/etc/sysconfig/network-scripts/ifcfg-eth0 in my case)

DOMAIN="domain1.org domain2.org domain3.org"

After either using nmcli or manually editing your file, simply restart the network service and your search domains now work!