Hello Mélanie,
this is currently (DW 7.9) not possible. A document can appear only once in a folder structure and therefore keyword fields can not be used for that. However, there are some ways around that.
If the keyword only has a few fixed options like "Bank A", "Bank B" and "Bank C", you can create one folder structure per option and use the "Filter" feature with "Keyword Field Contains Bank A" for example.
Another Workaround would be to use a single folder structure and a text field, then store N copies of the document, N being the number of different entries in the keyword field. Automatic copying of the document can be achieved using a custom application which connects to the REST API. I implemented this for some of my customers.
A Workflow might also work, because the REST API can be used from inside a WebService activity starting with Version 7.9.
The last option would obviously be to reconsider the use case. Perhaps the customer only wants this scenario implemented because they are used to it, but using search dialogs instead of folder structures might remove the need for this completely.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant