This UDF will add the leading zero to the number based on the length of the field.

addLeadingZero.java

public String addLeadingZero(String FieldValue, String Zeros, Container container) throws StreamTransformationException{
	String newFieldValue = "";
	String newZeros = "0";
		
	try{
		//Convert Zero to int iZeros
		int iZeros = Integer.parseInt(Zeros);
		
		//Check how Zeros to add
		iZeros = iZeros-FieldValue.length();
		
		// If Zeros length is less than actual FieldValue length
		if (iZeros <= 0 ) return FieldValue;

		//Run the loop to Create Zeros String
		for(int i=0; i< iZeros-1; i++){
			newZeros = newZeros + "0";
		}

		// Append Zero String to the actual value
		newFieldValue = newZeros + FieldValue; 
			
	}catch(Exception ee){
		newFieldValue = FieldValue;
	}
		
	return newFieldValue;
}