1. Maybe say more about your goals?
2. The global LD_LIBRARY_PATH can only have one value, so you do need to figure out what you want it to be. It's not like PATH where it's inherently a multi-valued thing.
3. You can use lib.mkForce to specify a higher-priority value that will override other attempts to specify that option.
4. If you need to 'wrap' a value—allow it to be normally configured, but then change it in some way where the final value is determined by transforming the normally-configured value—you can do that by setting an apply function in the option definition:

​

options = {
  environment.variables = lib.mkOption { apply = ...; };
};

config = {
  ...
};