Yes, you can.

The divine crusader specifically prepares spells—which is almost pointless for the typical divine crusader, as they only have one spell of each level. They could have metamagic feats that make the preparation non-trivial, but they aren’t really expected to. Or they could get more spells, e.g. through an extra domain. Complete Divine—the same source as the divine crusader itself—details how extra domains work, making it very likely that this interaction was anticipated but the authors. And those details say someone who can prepare spells (but isn’t a cleric) can choose to prepare spells from the extra domain in any slot (of sufficient level, of course).

Yes, you can.

The divine crusader specifically prepares spells—which is almost pointless for the typical divine crusader, as they only have one spell of each level. They could have metamagic feats that make the preparation non-trivial, but they aren’t really expected to. Or they could get more spells, e.g. through an extra domain. Complete Divine—the same source as the divine crusader itself—details how extra domains work, making it very likely that this interaction was anticipated by the authors. And those details say someone who can prepare spells (but isn’t a cleric) can choose to prepare spells from the extra domain in any slot (of sufficient level, of course).