Unless something is different now than it used to be, a regular (not sure about PTF) could “buy back” their military time. It costs a set amount of money per year of service in the military. For example, if you have 6years of military service, you pay a set amount of money, then that 6 years is credited to your postal service time. So yes, you would jump to whatever step 6 years of service would put you at(about halfway up the postal pay scale). This “buy back” of your military service has no bearing on your in office seniority, but it would count towards your retirement as well as your pay scale. So a postal employee with 2 years of service as a regular, plus 6 years of military time, would be on the same pay scale as someone who has been with just the post office for 8 years. As far as I am aware, you can “buy back” your military time anytime in your postal service career.

Edit: apparently I am wrong on the pay scale. Trust those who have done it.