Well, I made a mistake 
You should post to https://vco:port/content/workflows/{categoryId}
where categoryId is the id of the category where you want to import the workflow
Look here in REST API docs: https://vco:port/api/docs/resource_Content%20Service.html#.content.workflows.-categoryId-
This category id can be get using GET to https://vco:port/api/categories and look for target category ID
Look here in REST API docs: https://10.23.33.55:8281/api/docs/resource_Category%20Service.html#.categories
Hope this helps
Ivan