Star sub-network with LoRa capability?
Yes, it is possible to use a SX1272 or SX1276 to receive data sent by sensors and then use it to transmit data to a LoRaWAN public network while staying very low-power.

All LoRa transceivers have a "channel activity detect" (or CAD) function that can be used to save power and wake a receiver only when an emitter wants to send a message. Unlike traditional FSK transceivers such as the CC1310 that use RSSI, the CAD function is very selective and triggers only when a LoRa signal, with the proper bandwidth and spreading factor, is emitted in the channel. Because it doesn't wakes up every time there is noise in the channel, it can be quite power-efficient.

If you do a CAD detection every N seconds, each transmitter will have to send a packet with a N seconds preamble in order to wake up the receiver. This is a energy trade-off.