How to preserve the default value of a list?

I need(/want) to add "$HOME/.nix-defexpr/channels" to my NIX_PATH, because of home-manager. I know I can set it in my shell, but I want to do it with nix.nixPath. I did look thru the documentation, but I really wasn’t able to find anything other than the ++ operator which doesn’t really work the way I need. Essentially is there something similar to this:

export NIX_PATH="$HOME/.nix-defexpr/channels:${NIX_PATH:+:$NIX_PATH}"

Edit: Found the solution:

(Also yes adding the path was pointless, as it is already there, but I was getting errors about nix-shell not being able to find the home-manager channel file for some reason…)