Is a boost converter called as dc transformer?
If yes,why?
Does a dc transformer exist?
How can we convert dc in to dc step up with a transformer?

I'm not sure what a boost converter is unless you are referring to a "buck-Boost" transformer set-up...
Transformers do not work with DC voltage...
Can you tell us what your trying to do as there may be another way to accomplish
What you need...

Homework?