Geometry issue

Dear all,

I am designing an encapsulation for a chip and I need to create a flow path through it.
The starting surface of the flow path is a circular surface and it should finish in a rectangular surface.
I am trying to figure out how to do this but I have not had success on this.
If somebody could help, it would be great.

Attached there is a picture of the design and in red you will find the surface that I want to unify.

Best regards,
Accel