If you want to add a secondary IP address to a NIC already in use in Linux, and have that change only temporary. 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 in order 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 address to the same dev NIC, the second is considered as secondary.