So could we say that the Dor and Breath are somehow related as well?
Well, that's not specific at all, really. Of course they are somehow related, they are both derived from Shards of Adonalsium.
Nightblood is steel right? What if a feruchemist stored speed in it? Maybe nightblood could steal it, maybe the feruchemist would get a surprise when he/she tried to withdraw his or her speed. What it was used as a hemalurgic spike? Scary stuff, the possibilites are endless.
Oh, I got it. Nightblood has speed stored in it, then is stabbed through the body of an elantrian, into it's waiting hemalurgic host.
but Hemalurgi spikes steal allomantic powers, would it steal the powers of an Elantrian? Plus Nightblood would just suck it's new host dry if it were used as a spike.
Actually a hemalurgic spike can steel allomantic powers, feruchemical powers or just plain human qualities depending who kill with it and where it's placed int he recipient.
I knew that I just wasn't thinking, still Elantrian powers are a bit different.
It's important to know that, perhaps, while from our armchair view of the Cosmere, that Hemalurgy could theoretically steal these spiritual and genetic codes and give them to another, that it there might not exist certain places to stab the spike for certain magic systems. We don't know enough to know, either way.
Practically speaking, though, even if there exists a place where a Hemalurgically imbued spike can be placed into one of Endowment's creations, it may be impossible to find, given the viewpoint of humans. I'd argue it would even take a Shard a very long time to figure such things out.