Is there any way to either copy or import/export a news-filter?
it depends what you are trying to achieve.but you can always do this via SQL. Please provide a concrete scenario.
I would like to copy a filter used on one RSS feed to another, how do I set this up?
Filters can be assigned to several feeds. So just assign the filter to the other feed under
Tools -> News Filters...
Select the feed and press "Edit".
The assignment can be done there in the right hand panel.
Can I copy a feed in SQL? and How do I import it into QuiteRss?
Of course you can copy a feed via SQL.
But why do you want to maintan a feed twice in QuiteRSS?
The import can be done by an insert command on table feeds.
Unfortunately there is no such tutorial. Just provide the feed url and details about the filter you want to create.