If you want to add a secondary IP address to a NIC already in use in Linux, and have that change only temporarily. Enter this command:
ifconfig [nic]:0 [IP-Address] netmask [mask] up
An example is shown below
ifconfig eth0:0 192.168.1.2 netmask 255.255.255.0 up
You need to be root to execute that command.
Using IP command
If you prefer to use the ip command instead of ifconfig
ip address add [ip]/[mask-digits] dev [nic]
Here is an example
ip address add 192.168.99.37/24 dev eth0
With this command you can add more ip addresses to the same dev NIC, the second is considered secondary.