Sending 10 bits using arduino SPI

See section 8.5 of the datasheet - you send two bytes, but you don't pack them like that - it's 2 dont-care bits, 2 setting the mode, 10 data bits, then the two more don't care bits

So...

byte mode = //put the mode number here. 
byte firstbyte=(mode << 4)|(DATA>>6) 
byte secondbyte=DATA<<2

and then send the two bytes.