User-assignable “fields” are planned for a future generation of DEVONthink.
However, as the late Senator Russell Long said, there are more ways of killing a cat than by stuffing it with butter.
For example, you could create a new tag group called Publication Date. Make it hierarchical by creating new tag groups within it for each year of publication date that applies to your collection, e.g., Published 1996.
To view all of the documents that have a publication date tag (any year), switch to Tags View and click on the Publication Date tag.
To view just the documents published in 1996, click on the Published 1996 tag.
A similar approach works with group hierarchies, if in Database Properties you uncheck the option to exclude groups from tagging.
The full Search window (Tools > Search) has an Advanced button that allows tags to be included with other criteria in a search, and can create a smart group for any search by clicking on the “+” button to the right of the query field in the Search window.