Hi Yasmin Saad,
First add 2 fields in Membership module -> User - Settings class. This 2 field will be for this 2 dropdowns. Then you can create user control which will be your custom registration web part, where you will register user. U can create user with Create user . And third and the most difficult is to attach on separate database to populate this 2 dropdowns. You need to map fields form user control on user object and to register user.
Second solution is to create 2 custom forms controls where you will create dropdown with values from separate database. Then to add 2 new fields in Membership module -> User - Settings class, which will be this custom form controls, and then to use kentico out of the box registration web part.