No, not quite- sparge liquor MUST be boiled and I wouldn't add any after 15 minutes from the end of the boil to be sure it gets sanitised, there's also other processes at work (caramelisation etc) which make it important to get it in there as soon as possible.
If you're talking about plain water though, then yes indeed! I'll quite often add 7 L or 8 L of water to about 16 L of concentrated wort and that's the reason why this works- it is quite concentrated compared to ordinary BIAB, and closer to conventional 3V mashing. You could do it without the sparge, i.e. as per an ordinary BIAB, but it would be sacrificing about 25% of the available sugars, or most likely more, so would need a much heavier grainbill. So, I'm not talking your standard no- sparge/ full- volume BIAB, but about sparging the full- sized grainbill for an over- gravity boil and post- boil dilution and this is how I get bigger batches out of the smaller kettle _efficiently_.
With this method, I'm actually not that concerned about SG or volume targets, the reason is that with the post- boil dilute and with it all the fartarsing around with both volume and concentration targets along the way can be discarded in one fell swoop. I don't really care what the first runnings SG is, nor the sparge/s SG (but I always measure them, I'm a bit geeky- can you tell?), no, the only values I really need concern myself with are post- boil SG and volume, plus the target SG. Then I just need to calculate the amount of water to add:
Actual SG/ Target SG * Actual Volume = Final Volume
Then: Final Volume - Actual Volume = Amount of water to add!
SGs such as 1.055 are expressed as 55 in the formula, so imagine I have 17 L of 1.078 and want to dilute to 1.055:
78 / 55 * 17 L = 24.1 L
Then 24.1 L - 17 L = about 7 L water to add. Obviously this assumes no losses to trub...
If I exercised some discipline and focussed on finishing the guide I'm presently working on, all would become clear!

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Give me a beer and I will move the world. Archimedes