Adding water during boil to make a 90 minute boil possible?

So until my 8 gallon electric BK is ready, I'm using a smaller 5 gallon and BIAB. This weekend I'm brewing my first hefe, which also marks my first use of pilsner malt. Now, with a 60 minute boil, I know I'll be right to the rim of my kettle at mashout, meaning I'll be about 2.5 quarts low for my Total Kettle Volume on a 90 minute boil. Now, I understand the theory behind boiling pilsner for 90 minutes is to drive off DMS precursors, but which of the following options is the lesser of the evils?

1) Start with my "60 minute" boil volume, add the remaining 2.5q near the end of the 60m boil, and then boil for another 30 minutes, or

2) Take my chances with a 60 minute boil and no "top off"

Any opinions? Thanks everyone!

Boil a gallon of water before brewing, cover and add to fermenter(up to needed level)
I wont recomend adding cold tap water sinse i dont know your source

Topping off in the boil might be tricky sinse it will probably stop the boil

janivar: thanks for replying! My concern isn't about volume so much as it is about boil time; Plan #1 was meant as a way to extend the boil time another 30 minutes to drive off DMS precursors.

Sure, no problem adding water. I bring it up to boiling on the stove first, and likewise it doesn't stop the boil of the wort at all.

I try to save any sparge water after my boil kettle is full just in case I need it. It is better than adding plain old water. Right wrong or indifferent that's what I do.

Just wanted to update this in case anyone is looking for the answer in the future: on Brewday, I held back the "balance" of the water that wouldn't fit in the kettle, and at around 20 minutes left, I brought the "remaining" water to a boil and added it back to the kettle. Boiled for the total of 90 minutes, hit both my numbers and volumes exactly. Seems like it was almost too easy! Either way, I'm doing it again today