Do you mean add it on to the load from the shed or are you suggesting that you tap in to the SWA along the route?
What do you intend to power from the external socket? Whats the rating of the RCD and the other fuses?
If the shed service has been done correctly you should have a mini consumer unit with isolation and suitable fuses for the lighting and existing socket. If so, then it's a simple enough job to run a cable to the external socket position.
You could make the socket circuit a radial with the external one being the final, so a suitably rated hi-tuf or external 2.5mm cable between the shed socket and the external location will work.
Make sure you use the correct glands and on the external socket bottom entry is best.
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