I guess you are referring to CONVERT 08/01/2011 to 20110801, right
The best approach will always be to perform all data transformations in the database.
In your case it depends if 08/01/2011 is in date format : YYYY-MM-DDThh:mm:ss+HH:MM
If it is in date format then it is easy to convert it to the format you wanted as follows:
To convert to number you would do: <?xdofx:to_number('12345')?>
But on a second thought what is it the ultimate goal of converting it to number???