You shouldn’t confuse claiming a device to an account (which resides in the cloud and hence requires a cloud connection) and adding a device to a mesh network.
Also setting up a new network and adding a device to an existing network are two things.
I’m not entirely sure (and haven’t tested) whether extending a mesh network requires a cloud connection - if a connection exists, the node count and the “inventory” is maintained in the cloud (in parts to keep track of the size of the mesh for the micro network pricing threshold). Hence it might be compulsory.
I’m also not entirely sure whether devices of another account (and for that also unclaimed ones) would be allowed on the network of a particular gateway.
However, the way to create and extend a mesh network via USB would go via the
particle mesh commands
Manage mesh networks
Usage: particle mesh <command>
Help: particle help mesh <command>
create Create a new network
add Add a device to a network
remove Remove a device from its network
list List all networks and their member devices
info Get information about the current device's network
scan Scan for networks