Reg User Exit for Infotype 8

Hi,

I have created two custom fields on infotype 8, i need populate on fied with base salaryBonus(from infotyp 14) and the other field should be the total of all line items on Infotype 8 ( Base salary shift differentials(permiums).

In order to do this, i searching for user exit , can anyone help me in doing this ..please

Thanks

Kisu