You can build the routine...
Take the multibyte mathe routine, you need only MUL and ADD.
for example:
decimal 1576 convert to hex algo.:
X = 0
X = X + 1 first decimal
X = X * 10
X = X + 5
X = X * 10
X = X + 7
X = X * 10
X = X + 6 last decimal
now in X is the result in hex.

You can find the math routines in the Code Library.

Sascha