Default a submitter (uploader) of a document can add self chosen keywords to that document.
It is also possible to configure DSpace in a way that the submitter has to choose from one or more predefined keywords (controlled vocabulary).
The DSpace manual seems to suggest that you - when configuring - have to choose between free and predefined keywords.
I would like to give the submitter the possibility to choose between one or more predefined keywords. But also that he or she can add one or more self chosen keywords. Is that possible?
The hierarchical taxonomy feature gives you exactly this: https://wiki.duraspace.org/display/DSDOC5x/Authority+Control+of+Metadata+Values#AuthorityControlofMetadataValues-HierarchicalTaxonomiesandControlledVocabularies
You can see it in the demo installation on the "subject" field: you have a lookup feature that allows lookup in a tree of subjects, but manually entered values are possible as well.
screencast: http://screencast.com/t/0Cth3mORwxd