Well, this is not something I have done very often, .30-30 to .357 Herrett and .45-70 to .33 WCF are about it in my case forming career.
But since you are making your own dies, you can work up to the required set without too much trouble.
Make the sizing die first. Will it neck a .38 to 6mm in one step? If so, good.
If not, make up another and ream its neck area out until it WILL size a .38 down some amount, the exact neck diameter not important.
Will THAT somewhat bottleneck case size down to 6mm in the sizing die?
If not, make up another and ream its neck section out until it WILL size a case from the first die down by some more.
Will THAT more bottleneck case size down to 6mm?
If not, add another step and repeat until you have enough to bring it down.