Even if it's a bit tricky the White Willow can be obtained in this modpack, first you'll need to get the forestry versions of oak and birch (by placing them inside the portable analyser). Then you just have to breed them together either with bees or just using pollen collectors and a grafter. The tricky part is that to get the sapling you need to be in a biome with high temperature and humidity, an example would be the jungle luckily you can change your island's biome with /is setbiome jungle. The other mutations to get zebrawood should be trivial.
Also could you explain why you can't get the withering bee?