RegisterImport["format", defaultFunction]
register defaultFunction as the default function used when importing from a file of type "format".
RegisterImport["format", {"elem1" :> conditionalFunction1, "elem2" :> conditionalFunction2, ..., defaultFunction}]
registers multiple elements (elem1, ...) and their corresponding converter functions (conditionalFunction1, ...) in addition to the defaultFunction.
RegisterImport["format", {"conditionalFunctionsdefaultFunction, "elem3" :> postFunction3, "elem4" :> postFunction4, ...}]
also registers additional elements (elem3, ...) whose converters (postFunction3, ...) act on output from the low-level funcions.

First, define the default function used to import the data.

RegisterImport is then used to register the above function to a new data format.

Conditional Importer: