Both enabling ibus input method and installing the IM engine have their own NixOS options:
i18n.inputMethod.enabled = "ibus";
i18n.inputMethod.ibus.engines = [
pkgs.ibus-engines.bamboo
];
The way to actually enable it will depend on desktop environment. For example, for GNOME, you can add the input method in GNOME Settings (Control Center). Or do the same declaratively as in Strange xkbOptions behavior (GNOME) - #5 by jtojnar.