Hi Jeroen.
This is possible by using a combination of tables and snippets. First create a table with one cell in it and in the class of the cell put input-group.
Class on table cell:
Then create a snippet and place your text input first and then your label called days with the class input-group-addon second.
Then you have the desired functionality.I hope this helps. If not i can include some screenshots on how to do it.