add request.GET variable using django.shortcuts.redirect

Since redirect just returns an HttpResponseRedirect object, you could just alter that:

response = redirect('url-name', x)
response['Location'] += '?your=querystring'
return response

Is possible to add GET variables in a redirect ? (Without having to modifiy my urls.py)

I don't know of any way to do this without modifying the urls.py.

I don't have complains using HttpResponseRedirect('/my_long_url/%s/?q=something', x) instead, but just wondering...

You might want to write a thin wrapper to make this easier. Say, custom_redirect

def custom_redirect(url_name, *args, **kwargs):
    from django.core.urlresolvers import reverse 
    import urllib
    url = reverse(url_name, args = args)
    params = urllib.urlencode(kwargs)
    return HttpResponseRedirect(url + "?%s" % params)

This can then be called from your views. For e.g.

return custom_redirect('url-name', x, q = 'something')
# Should redirect to '/my_long_url/x/?q=something'

We can import urlencode from django.

from django.utils.http import urlencode

get_args_str = urlencode({'q': 'something'})

Or we can just use unparsed get parameters string from starting request

get_args_str = request.META['QUERY_STRING']

HttpResponseRedirect('%s?%s' % (url, get_args_str))

I think it's worth noting that Django's RedirectView class has a built-in class attribute query_string which can be overridden or passed to as_view. If set to True, query_string will preserve the query string in the redirect. For example, you could put the following in urls.py:

path('example-redirect/',
     RedirectView.as_view(url='https://example.com', query_string=True),
     name='example_redirect')