Yes, a woman is allowed to prepare a specific meal, such as a packed lunch, for her husband while she is niddah.
Serving one's spouse is considered to have special overtones of affection and is therefore subject to certain restrictions during niddah. Thus, a wife should not place food on her husband's plate in front of him, nor prepare a plate of food and set it directly before him. It is fine to bring a platter to the table "family style," from which he also takes.
According to some authorities, these restrictions on serving are limited to drinks; according to others, they apply to food as well. They are reciprocal – the husband also cannot serve his wife.
For more discussion of this topic, please see our article Affectionate Service.