Add a new function to add to request methods (POST, GET etc)

It's not super clear in the doc, but what you're looking for is setGlobal:

PHP Code:

$this->request->setGlobal('post', [
    'foo' => 'bar',
    'bar' => 'baz',
]); 


Yes I saw that, but that overwrites the values of all other $_POST variables.