It can't! But you can keep the keywords window open as you're working. if I'm doing a batch of keyword adding, I tend to tuck the keyword window at the bottom of the Inspector, then minimise the General Meta-Data and Synopsis panes. This makes the keywords pane pretty large, plenty of room for adding keywords, while keeping the keywords window open too, even if it's enlarged so you can see all your keywords.
I don't think there's a way of hiding the keywords window behind Scrivener.
Yes, I know. When I was referring to the 'keywords window' I meant the window you show in your screen shot. When I referred to the 'keywords pane' I meant the pane displayed in the Inspector which lists the keywords attributed to a certain file. I like to keep both open so I can drag and drop keywords from the 'window' to the 'pane', and I find there's enough room to do this if I minimise the other elements of the Inspector. It works for me, but maybe it doesn't suit your way of doing things?