Just got an answer from Rak Wireless with a very easy solution:
As you mentioned TTNv2 and TTNv3 use the same packet forwarder frontend. To be able to use your gateway, you only need to update the URL in the global_config file to the new TTNv3 URL, based on the location you are.
We are preparing a BasicsStation firmware also, but this will take some time.
To change the packet forwarders global_conf file URL:
1. Login to your gateway with SSH(console SSH or PuTTy)
2. Use the command `sudo nano /opt/ttn-gateway/packet_forwarder/lora_pkt_fwd/global_conf.json` to open the config file with the Nano editor.
3. Find the "server_address" field
And change the old URL "[router.eu.thethings.network]" with the new one for TTNv3 - EU for example: "[eu1.cloud.thethings.network]"
Save the configuration with Ctrl+x command, and confirm the changes.
4. Restart the packet forwarder process with `sudo systemctl restart ttn-gateway.service` command.
If your gateway is correctly registered in TTNv3, after a minute and a TTNv3 page refresh, you will be able to see your gateway connected.