How to create "New Expression" that can be used in Condition Editor for an "Action"??

Dear Experts,

I Know that while creating Start Condition for any Action we go to "Edit Condition" and then we select Expression 1, Operator and Expression 2 from the list of Expressions given under a conainer.

My Question is what if I do not find a suitable Expression? Do I need to create a New Expression? If yes how can i create a New Expression and make it Selectable?

Points will be rewarded gladly. Please help. If posible wih some Eample/screen shots etc.

Regards

Satyajit