A rule set can include any number of individual rules, and you can add new rules to a
folder as you need.
-
Hover over a folder with rules applied (indicated by the
icon) and click 
More then Manage Rules.
-
Click New Rule.
The New Rule page appears where you can add a new rule to the folder in exactly the same way as the first time you created a rule - see Create a rule from scratch for more details.
After creating the last rule you return to the Rules page. Any new
rules created are added at the end of the rule set.