How do you override USART1_Handler without editing variant.cpp ?

Dear forks,

variant.cpp is arduino-1.5.2\hardware\arduino\sam\variants\arduino_due_x\variant.cpp

I want to use TXD1,RXD1,SCK1 as SPI slave mode with interrupt function.
Because of USART1_Handler is defined in variant.cpp,
multiple definition is occured when I define USART1_Handler in my own code.

Do you know smart code about this issue ?