DECODE is an SQL function, not PL/SQL. You can not use it in plain PL/SQL, only as part of SQL statements.
There is no nice and easy way for this. There is the method with collections, for example:
create or replace type t_numbers as table of number;
create or replace function sum_plsql (p_numbers t_numbers) return number
rv number := 0;
for n in 1..p_numbers.count
rv := rv + p_numbers(n);
select sum_plsql(t_numbers(1, 1, 1, 1, 1, 1, 1, 1, 1)) from dual;
There are built-in, non-documented simple list types, so you do not need to define your own types at all circumstances, but relying on non-documented features is generally bad practice, as they may change in newer versions without warning.
If there is a reasonable limit for the number of arguments:
create or replace function sum_plsql2 (p1 number default 0, p2 number default 0, p3 number default 0, p4 number default 0) return number
return p1 + p2 + p3 + p4;
select sum_plsql2(1, 1, 1) from dual;
Obviously, these are neither tidy nor effective.