You need to replace array of point with object, like:
series: [{
data: [{
x: 67,
y: 78,
z: 20,
customParam: 'custom text1'
},{
x: 14,
y: 68,
z: 50,
customParam: 'custom text2'
},[20,20,20]]
}]
And in the tooltip get parameter form point.options
tooltip: {
formatter: function () {
return this.point.name + '<br/><b>' + this.x + '</b><br/><b>' + this.y + '</b><br/>custom text: '+this.point.options.customParam;
}
},