Here is a example of how to set up one IP alias and help to create a new one easily.
In the IP alias dialog, define a full range of IP aliases. This will likely be sufficient for quite some time. Suppose your eth0 network is 192.168.1.0 and there is an unused section. Just enter
192.168.1.50-100
which will allocate 51 IP aliases for future use.
Now, in the "Configure domain name server" menu, you select the "IP allocation space" entry. Enter the range and a small description such as "Virtual domains." The DNS is not always on the same machine where you define the IP aliases.
Go into the DNS and add a host for the domain, generally www.new_domain. For the IP number, hit ctrl-X and Linuxconf will compute the first unused IP number in the IP aliases range. Pick this number.
Then go into the httpd configuration and add a section for the new www domain. Repeat this step for each new domain until you are out of IP aliases.
Using this strategy (defining a bunch of aliases first, telling the DNS about them next, and using them last), you will avoid many errors.