ActionScript 2.0 :: Push An Entry Into An Associative Array?

How do I push an entry into an associative array?

ActionScript Code:
var lijst:Object = new Object({fname:'John', lname:'Carpenter'});
lijst.push({mname:'Frank'});

[code]...

This doesn't work. The first name and last name work because they are created when the object was created, but the pushed middle name remains undefined.I understand that associative arrays are in fact objects. I tried turning it into a real array so the Push command would work cause it's part of the Array Class.

ActionScript Code:
var lijst:Array = new Array({fname:'John', lname:'Carpenter'});
lijst.push({mname:'Frank'});

[code]....

Though this works, it only works when using a '1'. When in fact I want to keep it inside the '0' entry.So how do I really add an entry into an assciative array? So that in above example instead of each having two elements one is added resulting in three elements per entry?