NodeJS add element to Object

I need to add element to existing Object.

Exemple of Object (Mongoose):

param: { 
    hello: 'Olá Mundo PT '
}

And I need to add new element, ex :

params.param.set(JSON.parse('{"'+req.param('param')+'":"'+req.param('trans')+'"}'));

//And I tryed push

 params.param.push(JSON.parse('{"'+req.param('param')+'":"'+req.param('trans')+'"}'));

But nothing works...