You can apply the same keyword selecting several documents and then using the contextual menu. I do not know if it might help you in some way? Otherwise, you should build your own UI for it.
Also, I do not suggest using keywords for it, I suggest use metadata rather than keywords for performance and other reasons that will be so long to explain. Basically, create a field definition into metadata for storing keywords rather what are you doing now. With metadata, you have the same option for adding fields or updating to several nodes.
Take a look here:
https://docs.openkm.com/kcenter/view/ok ... adata.html