The safest way to create your own variables is to create your own xxx_variables weblet and import this wherever you need access to the new variables.
You can use the existing std_variables weblet as a guide but don't attempt to redefine variables already defined in std_variables. This is not necessary and may result in strange and unexpected behavior.
Some knowledge of XSLT, and a thorough understanding of where and how variables are used, is required before attempting to create your own variables.