How to add multistep registration form in default user registration
You can try Field group


  Fieldgroup will, as the name implies, group fields together. All
  fieldable entities will have the possibility to add groups to wrap
  their fields together. Fieldgroup comes with default HTML wrappers
  like vertical tabs, horizontal tabs, accordions, fieldsets or div
  wrappers.


Available group types

Fieldsets
Horizontal tabs
Vertical tabs
Accordions
Divs
Multipage steps: Note: This is only client side.


The idea is to use Multipage steps to make the form steps.