Add all songs from Top Charts to a playlist
This would be a neat feature, but there is no way to do this.

And, to be honest, I think it'd be tricky to implement without also providing the user a number of options they need to select.

The problem is that songs in the top charts are constantly changing, so the user would need a way of choosing how this is managed? 

For example, let's say it's a Top 50 chart:


Once the songs are downloaded, do they remain downloaded when they are no longer in the Top 50? 
Should the playlist auto update when the Top 50 changes? 
If not, how long should the playlist exist before it does check the Top 50 again? 
What about the ordering of songs? Should they remain in Chart order? If so, should the playlist reflect the current order at any given point of time?


The list goes on...

However, the idea does have merit. I suggest you send Apple feedback about this. I know they're focusing quite strongly on Apple Music etc and something like this is certainly worthy of consideration. I'd make sure your feedback covers the fact that you've considered some of the issues of implementing this and, if possible, suggest how you'd deal with those.