Possible to share mobile internet (tether) while connected to existing wifi network? I have an existing wifi network and a Google Nexus 5 (un-rooted, stock android) with 3G data connection.

I would like to tether my mobile to share internet to all devices in existing network which have a custom route or proxy, maintaining default route for all other devices. This existing network also has another fixed connection (default route).

This means Android should effectively connect as a guest to existing wifi network, maintaining 3G/4G data connection and allow data to traverse through its mobile network while providing Gateway / NAT function for any devices which have an appropriate route / proxy set.

Configuration for the other devices (setting routes or configuring proxy) can be managed by hand (manually) as I am a capable network administrator.

As this is typically a consumer device, is this possible? and can anyone provide suggestions on software / howto's / information for this? Technically, my knowledge of computers networks and linux tells me this should be 100% possible, and relatively simple to achieve.

The cost of data here is not in question, I understand that connecting a mobile to an existing network could use a substantial amount of data and may impact my contract and data allowance.
Maybe I am not understanding correctly, but you could set your phone as a Mobile Hotspot and configure a wireless router as a bridge between your existing network and the hotspot. In this scenario, all devices on your network would be using the wifi signal of the router and only the router would need to be configured to reconnect to the hotspot.