FWIW in bash I have 2 functions:

  path_add() {
      export PATH=$PATH:$(string_join ':' $@)
  }

  path_prepend() {
      PATH=$(string_join ':' "$@"):$PATH
      export PATH
  }

These can join an arbitrary list of paths to PATH. e.g.

  path_add /usr/bin /usr/local/bin ~/bin

They depend on another one:

  string_join() {
      local join=$1; shift
      local result=$1; shift
      for p in "$@"; do
        result="${result}${join}${p}"
      done
      echo -n "$result"
      set +x
  }

I also have ones for adding and prepending to LD_LIBRARY_PATH