Adding Static routes within vRNI Appliances is not recommended/supported, however, below are the steps, if you want to implement.

Why would we need this?
Static routing can be used to define an exit point when no other routes are available or necessary. Static routing can be used for small networks that require only one or two routes.
Scenarios wherein there is a network limitation which force us to have 2 Gateways, we can add Static routes accordingly.

How do we implement?
Browse the file /etc/network/interfaces

Add Static Routes accordingly as mentioned in the below screenshot