Natural processes external decimal (format N) values just like any other mainframe language would. That is, the right-most byte contains the sign in the left nibble. Here's some code to demonstrate.
DEFINE DATA LOCAL
1 #N (N3.4)
END-DEFINE
MOVE -234.5678 TO #N
WRITE '=' #N #N (EM=HHHHHHH)
END
And here's the output.
Page 1 12/14/15 00:47:05
#N: -234.5678 F2F3F4F5F6F7D8
Notice that the field definition is N3.4; 3 digits before the implied decimal point, 4 digits after.