Yes, and the arithmetic is identical to diluting wort before fermentation — but what's actually being diluted has changed, and that matters more here than it does earlier in the process.
The same gravity-points math, applied after the fact
18 L of finished beer reading 1.055 topped up with plain water to a 1.050 target needs 1.8 L of water added, landing at 19.8 L total. Check this exact example — the method is the same gravity-points calculation used anywhere else on this site: finalVolume = currentVolume x currentPoints / targetPoints.
What's different once fermentation is done
Before fermentation, diluting wort lowers the sugar concentration that yeast will eventually convert to alcohol — the final ABV shifts because there's proportionally less fermentable extract per litre for yeast to work with. After fermentation, the alcohol is already made, and diluting the finished beer lowers its concentration directly and immediately, the same way adding water to a spirit lowers its proof. A beer at 5.3% ABV diluted by roughly 10% in volume (the ratio in the example above) comes down to roughly 4.8% ABV — nearly the same proportional drop as the volume increase, because alcohol itself is what's being spread across a larger volume now, not future fermentable sugar.
Why you'd actually do this
The most common real reason is a batch that finished stronger than planned and the brewer wants to bring both gravity and ABV down closer to a target style before packaging — a legitimate, well-understood fix, not a workaround. It's also sometimes used to stretch a small batch to fill an extra bottle or two, though that comes with the same proportional strength trade-off. What it will not fix is a flavour or process problem — watery character from over-thin brewing, an infection, or an off-flavour from fermentation — since water only ever dilutes concentration, it doesn't remove or correct anything already present.
The trade-off worth knowing before you do it
Diluting finished beer also dilutes everything else already in solution — hop bitterness, colour, body, and any subtle character the beer developed during fermentation — proportionally along with the alcohol and gravity. A beer diluted enough to meaningfully lower a too-high ABV will taste noticeably thinner across the board, not just less alcoholic. If you want gravity and ABV down without also thinning out colour and bitterness, blending with a second, weaker batch instead of plain water lets you control those independently — see dilution vs. blending: when to use which for the fuller comparison between the two approaches.