I think this does it:
Code:
Input VAR BYTE
Wide VAR WORD
X VAR WORD
Y VAR WORD

EnlargeYourByte:
X = Input
Y = Input << 4
X = X ^ Y
X = X & $0F0F
Y = X << 2
X = X ^ Y
X = X & $3333
Y = X << 1
X = X ^ Y
X = X & $5555
Wide = X << 1
RETURN