help replacing expresion in plpgsql

Hi, i have a function that receives a parameter which represents days:

FUNCTION aaa_recharge_account(expdays integer)

i want to add those days to the CURRENT_DATE, but i do not know how to do
it, i have tried several ways to replace that in an expresion like:

newexpdate := CURRENT_TIMESTAMP + interval '$1 days' using expdays;
(newexpdate is declared as timestamp)

and many more but none work, can someone please help me to find out how can
i replace that parameter into an expression that i can add to
CURRENT_TIMESTAMP or any other way that i can accomplish what i need which
is to add that parameter to the current timestamp. thanks!!!