No direct conversion possible as a cursor is a program (not result set) and an associative array is a data structure.
Code is not data. Data is not code. In most cases. Including this one.
Also an associative array does not make that much sense in a database environment. It does not scale as it
a) resides in private process memory (PGA)
b) is non-shareable (each process/session needs their own duplicate copy for the same data)
There is very little real world use for associative arrays in PL/SQL. In my view, when using an associative array in PL/SQL you are very likely doing it wrong - and using the incorrect data structure for the task at hand.
You do not "convert" a sys_ref_cursor to an associative array. You may just fetch all it returns into an associative array - that is if you may be certain that it does not fetch a great deal of records.