Extend SalarySlip class

Hi. I want to add some variables that can be used on formula on salary structure, let said total_sick_leave.
How I can do that? I was able to add it on SalarySlip class directly, but it was on core file, I want to add it on custom app, can I do that? like extend SalarySlip class to add some custom field with some custom calculation.

Thanks.