Hello,
It is possible to add multifield with richtext inside content fragment (with some custom node configuration from crxde) as it uses the same dialog resource type cq/gui/components/authoring/dialog
i just did it and it works with multifield after configuring the content fragment model using (contentreference widget --> and configure it for multifield) default will come with text field. Now go to your model its created for example conf/project/setting/dam.... and then change the configuration of "field" node under multifield node as attached image
note: make sure the mulfield node and field node has same "name" configured as it comes already configured when you create model initially.
hope it helps!!

