How: obj-class-A = ? suppose class A is subset of class B

I mean create an object a-A by another object b-B.
a-A is object of class A, b-B is object of class B.
suppose A is 'subset' of B, em, I mean, all the attributes A have, you
can find attributes with the same name in B.
How can I gain this? re-write the 'new' method in A? or just define
another method create_via_B(b-B)? so how ?

I'm not sure if I fully understand what you're trying to describe but
it sounds like you want to look into Single Table Inheritance (STI).