You could use lemon to parse that custom BNF grammar to turn it into a grammar that lemon likes.

That being said, based on my playing around with lemon, the grammar is only a small part of the work with getting the actions right taking up the bulk of the time. You have to insert a bunch of attribute indicators (for lack of a better term) that get passed into the actions anyway so you can’t just take a perfectly formed grammar and use it without a bunch of work.

Plus some parser generators like left-recursive, some like right-recursive, some have ‘sugar’ to indicate repetition and/or optionality, some don’t so you have to specify it manually using empty rules, lots of different ways to do the same thing.

I did something like this here https://github.com/mingodad/lalr-parser-test where I expanded byacc/bison/lemon to generate a naked grammar, ebnf grammar (understood by https://www.bottlecaps.de/rr/ui to generate railroad diagrams) and interchange the grammar between then (byacc/bison to lemon and the other way around too, taking in account the difference in how they interpret rules precedence).

FWIW tree-sitter has done a lot of the ground work for this sort of thing, if you need to support a lot of different languages.

Why not just use the parser generator to generate a parser that will standardize the grammar and then use the parser generator again on the output of that parser to generate a parser for the desired grammar?

Its all parser generator all the way down.