Yes you can - technically you can switch the port on the Raspberry Pi to LAN and connect it to the WAN Port in Deco (PiFi App > Settings > Network Settings > Port (LAN/WAN) - Set to LAN) and pair wirelessly to Sky via PiFi app
But, what I’d recommend for best speeds is keep your Raspberry Pi plugged into your Sky router and connect a USB-to-Ethernet adapter (one like this) and have that cable go into the WAN port on Deco
That way you’ll get way faster speeds - I’ve seen close to 1Gbps VPN with that setup on my network!