How to add a subtitle field to a block view?
The easiest solution is to add a Global: Text area in the HEADER and type your Subtitle there.



You can use FULL HTML and wrap it in a div class or ID so you can do some CSS work on it. 


  Can i add additional fields just like with custom blocks?


You can add fields to your view



or attach another view that contains fields in the FOOTER, but fields in views and fields in custom blocks are totally different.