replacing expresion in plpgsql

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!!!