You can use the Concat db function.
from django.db.models import Value
from django.db.models.functions import Concat
ATable.objects.filter(id=100).update(some_field=Concat('some_field', Value('more string')))
In my case, I am adding a suffix for facebook avatars URIs like this:
FACEBOOK_URI = 'graph.facebook.com'
FACEBOOK_LARGE = '?type=large'
# ...
users = User.objects.filter(Q(avatar_uri__icontains=FACEBOOK_URI) & ~Q(avatar_uri__icontains=FACEBOOK_LARGE))
users.update(avatar_uri=Concat('avatar_uri', Value(FACEBOOK_LARGE)))
and I get SQL like this (Django 1.9):
UPDATE `user_user` SET `avatar_uri` = CONCAT(COALESCE(`user_user`.`avatar_uri`, ''), COALESCE('?type=large', ''))
WHERE (`user_user`.`avatar_uri` LIKE '%graph.facebook.com%' AND NOT (`user_user`.`avatar_uri` LIKE '%?type=large%' AND `user_user`.`avatar_uri` IS NOT NULL))
The result is all image URIs were changed from http://graph.facebook.com/<fb user id>/picture to http://graph.facebook.com/<fb user id>/picture?type=large