The menu command Plot Settings=>Edit Plot Defs File allows you to enter your own function definitions and parameter definitions for use in the waveform viewer. These functions are kept in the file %HOMEPATH%\Documents\LTspiceXVII\plot.defs.
Then the syntax is the same as the .param and .func statements used for parameterized circuits. E.g., the line
.func Pythag(x,y) {sqrt(x*x+y*y)}
defines the function Pythag() to be the square root of the sum of its two arguments.
Similarly, the line
.param twopi = 2*pi
defines twopi to be 6.28318530717959. Note that it uses the internally defined constant pi of the waveform viewer.