Modify Existing Form

Hi,

I am planning to make some modifications to the HR form and wondered the best way to approach this. I do not want to use the user defined fields form as that is too basic.

Adding the items to the GUI is straightforward enough, but what is the best approach to store the data and tie that data to the Employee item? Should I create user defined fields anyway then hook on to them with the new GUI items? Is there any good examples available of modifications to existing forms?

Thank you.