Wait so you could potentially block expansions or do I misunderstand?

Potentially, yes, but it would involve building a highway of creep from your base to the expo, which would take a lot of energy from a Queen that could otherwise be spawning you extra larva! Besides, an enemy can easily destroy the one or two Creep Tumors blocking it and wait for the Creep to recede.

And what if u excrete creep with an overlord wich carries a queen wich places a creep tumor on excreted creep?
Free and fast expansion block!
I succesfully get my password back

Yep, you could do that. But note that the Queen is much slower off Creep than on it. But hey, Overlords are pretty slow, too, so you'd be waiting for that anyway.

do overlord can carry ? or its the the overseer

Yup, Overlords can transport units after they've gotten the Transport Upgrade. In fact, I don't think Overseers can transport units at all.

i think overseers will because overlord can't have speed upgrade because blizzard make the overseers is a speed ovi and overlord is a slow ovi. So slow ovi drop suck, will we be force to use only nydus worm

lol I can imagine a slow overlord drop. It would be like "You have underestimated my element of... SURPRISE! ... okay, maybe I spoke too soon..."