I am really sorry to say that, in dokan frontend there are no such kind of feature. It is need huge work to do for this feature and very complicated work. But we have this plan, hope we will implement this feature near future.
That's a shame. when I asked the same question in pre-sales, before I paid for Dokan, I was told it was a very simple fix to add to more than one category. I hope its something that will be looked at soon, I'm sure others would need this feature too.