Firstly, I wouldn't do it this way - keep the long-term state in a lookup file. In case you have any mistake in your processing, you lose your history. So overwriting blindly your only copy of calculated stats is a bad idea.
Anyway, I did a simple test - reading from a lookup, modifying the value(s) and writing it back seems to work ok.
Keep in mind though that each run of such search would append your lookup contents in this form so you'd have to implement some fancy logic to check whether to add whole next row or just update one of the results and so on. There's no good reason to complicate things that much. I'd just stick to calculating results on the fly and accelerate the report if needed.
OK. It seems more complicated because the test did work (albeit run on a small all-in-one environment) but I wanted to do a one-off modification of a saved lookup and it seems doing an inputlookup then adding some column and saving the lookup file back didn't work. Strange.
It must have something to do whether I run it on all-in-one or on search head cluster, because it's repeatable - on aio it does work - modifying values, adding columns and so on. On SHC it seems to be not working.