Add entry with the select-entry-modal-field So I have a 'Single' that uses entries from different 'Structures'. I can choose one or more entries with the fieldtype. But often I want to select an entry that is not created yet .. so I have to go back create that entry and then select it .. 

It would be so much easier if I could just say "create a new entry" right there 

What I mean

Is there a plugin for that or anything ?
There isn't, as far as I'm aware of, but have a look at the Craft Plugin Library. If you write your own, you might have a look at the following to see an example of a modal that craft does use:

var modal = Craft.createElementSelectorModal('Entry');


Try typing that into your console on any page in the CP. 

Categories use a different kind of model when double clicking a category item that actually saves data, so might have a look at that as well.

Also, the amcommand plugin provides a nice modal command palette with a command to create entries that also might be worth checking out.