How can I extend an existing request?

The 
expects() method can be used to add custom validation rules (and messages) for your custom field (or to overwrite the existing rules/messages for an existing field):
// Adding a custom field
\Aero\Admin\Http\Requests\Catalog\CreateProductRequest::expects('acme', 'required|string|max:255', [
    'acme.required' => 'The acme field is required.',
]);
// Making existing categories field required
\Aero\Admin\Http\Requests\Catalog\CreateProductRequest::expects('categories', 'required|array');
Parameters
Parameter
Type
Description
$field
string
The key of the field to validate.
$rules
string|array|\Closure
The rules to use when validating. If callback is supplied, it will receive the request as the argument.
$messages
array|null
The messages for the rules, key's are of the form 
{$field}.{$rule}.