If an interface is not in
/etc/network/interfaces, but you want it to be there, then why don't you just add it there? It is a text file, the kind you can edit.
The standard tools for manual network interface configuration are
ip from iproute2 and/or
ifconfig (along with
arp) from the deprecated net-tools. They both allow you to change IP addresses, routes, and such. For automatic configuration via DHCPv4, either
dhcpcd can be used.
(Most of these programs are in
/sbin, so look there if you get a "command not found".)