Blizzard has stated you can supply only some materials and that collaborative crafting will exist for rare materials. It would defeat the purpose of being able to provide some of your own items if it doesn't work for the higher up crafting things as they also said that you can have soulbound items crafted through the work order system.
From the deep dive write up on mmo-champion https://www.mmo-champion.com/content...-System-Revamp :
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So the same as now with BoE items. This isn't a system that favors the elite and Blizzard has never said that raids will be the only source of materials. It also ignores how LFR will likely reward any BoP raid item.
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Anybody can raid. There is no gate keeping involved since materials will have quality levels and Blizzard can easily have different tiers to craft different raid level rewards.