Extra charge can be bought as a non-charge hero like abrams or warden

[f5]

If you queue it in to quickbuy with the option "Auto-buy items when within range of shop" enabled, it will be bought even though it appears to be locked. I do think that works with rapid charge too, but i didn't test that.

You can't buy headshot booster as graves using this method, it only works with extra charge for some reason.