Since Bugzilla 2.20 RC1, legal values for Operating Systems, platforms, bug priorities and severities can be edited from the User Interface directly. This means that it is no longer required to manually edit localconfig. Starting with Bugzilla 2.23.3, you can also customize the list of valid resolutions from the same interface.
Editing legal values requires "admin" privileges. A link named "Field Values" is visible in your footer and clicking on it displays the list of fields whose values can be edited.
You can add as many legal values as you want, and each value must be unique (on a per field basis). The sortkey is important to display these values in the desired order.
You can also delete legal values, but only if the two following conditions are respected:
The value is not used by default for the field.
No bug is currently using this value.
If any of these conditions is not respected, the value cannot be deleted. The only way to delete these values is to reassign bugs to another value and to set another value as default for the field.