For add multiple wifi networks / connections to volumio config you must edit wpa_supplicant.conf .
In terminal :
sudo nano /etc/wpa_supplicant/wpa_supplicant.conf
ctrl_interface=/var/run/wpa_supplicant
network={
scan_ssid=1
ssid="Wifi_A"
psk="Wifi_A_password"
priority=1
}
network={
scan_ssid=1
ssid="Wifi_B"
psk="Wifi_B_password"
priority=2
}
network={
scan_ssid=1
ssid="Wifi_C"
psk="Wifi_C_password"
priority=3
}
sudo wpa_cli -i wlan0 reconfigure
Hope you enjoy this settings!