You're all set with the 5m extension because as someone already mentioned one simply plugs into (a water tight seal) the other. I'm assuming a similar setup at the mast step as was on my C320 when I did it being a big blob of sealant at the top of the compression post. I removed that blob, fed the cable up through with a foot or so hanging out to connect with the extension coming down the mast, and re-made the sealant blob at the step. The extension even if not needed is beneficial because it gives you the water proof and electrically correct connection at the mast step for mast removal.
Wickford/Narragansett Bay RI