Is it a correct sentence? Can I use "into there" in this context?
"We travel through the time into the black hole, let's come back home into there."
Your sentence is not quite correct, breaking it into two parts
We can travel through time in that black hole...
We can travel through time with that black hole...
assuming you are already at home and want to come back at a different time
let's come back home through / in there.
if you are on the other side of the galaxy and want to get home
let's go back home through / in there.
One problem is that we do not know where a Black Hole leads to and current understanding of references (to, from, into) in Newtonian space might change once we do know what happens in relativistic space.
It's like the joke about twins in the womb
Twin 1: What is that?
Twin 2: It's a birth canal.
Twin 1: Where does it go?
Twin 2: We don't know, no one's ever come back.
Yes, but not in that particular sentence:
We can travel through time by using that black hole! Let's go back home by traveling through there.
Although you can say:
We can travel through time by using that black hole! If we go into there we will be home when we come out the other side.
(Leaving aside the fact that black holes actually have no "other side", of course :)