//Retrieve current query strings:
$currentQueries = $request->query();
//Declare new queries you want to append to string:
$newQueries = ['foo' => 'bar', 'popular'];
//Merge together current and new query strings:
$allQueries = array_merge($currentQueries, $newQueries);
//Generate the URL with all the queries:
$request->fullUrlWithQuery($allQueries);
Worked for me, added the parameters to the existing ones.
(Running Laravel 5.7)
Which is especially nice for usage within blade templates in a concise manner. Adding an additional country filter to the existing ones there to find users in the same country:
<a href="{{ request()->fullUrlWithQuery(['country' => $user->country_code]) }}">
All users in {{ trans('countries.'.$user->country_code) }}
</a>