Since in this case the changes may be limited to adding fcitx to the ackage function and patching the Cmakelist.txt or adding a -D flag or something, you might as well just modify the package in nixpkgs and file a PR.
How to do this is pretty well documented here.
And until the PR is accepted and merged, you can run your own package “in production” by adding an overlay like so:
nixpkgs.overlays = [
(final: prev: {
openbangla = (import
(prev.fetchFromGitHub {
repo = "nixpkgs";
owner = "yourgithubuser";
rev = "yourbranch";
sha256 = "sha256-crKfXyoYUzA6zwMdnor1Aw3DuiT8/PUiwn9zAiArOpg=";
})
)
})
]
(You’ll have to update the sha256 with the expected value which you’ll see in the error message when you try to build it the first time)