I can't find an easy way to transfer per2per (peripheral to peripheral) in atmel's data sheet. There is a PDC, but it's only supports per2mem:
The Peripheral DMA Controller (PDC) transfers data between on-chip serial peripherals and the
on- and/or off-chip memories.
Another feature, closest to bare metal DMAC doesn't include ADC in it's supported channel list.
Probably, it coulld be done by programming two independent PDC channels, one for ADC->mem trasnfer, and another mem->USART. So, management two channels pointing to same memory buffers may be tricky. Last time I tried to program mem->USART PDC, I had to overwrite an arduino core files, to "loose" USART from Serial predefined interrupt sources.
http://forum.arduino.cc/index.php?topic=267115.0