Mixing into the liquid resin won’t work. I’m thinking maybe lightly sprinkling the resin with the dry ground spices, when it’s at the gel stage would work, as long as they stay at the surface. The color of the spices should blend with the “gingerbread” look. The scent of the spices will fade in time. If painting the trim, a gingerbread scented oil might be blended with the paint, but I’m not sure this would not peel off later.