Well, once an analog signal is digitized it is no longer analog, so you cannot condition an analog signal when it is already digitized since it does not exist anymore...
But I think the background of your question is more serious. Basically you can simulate all kinds of filters, amplifiers and other signal conditioning circuitry by software. But you cannot avoid errors inherent to digital conversion of analogue signals, such as errors related to aliasing or overdriving an ADC input. So in many cases it is a better approach to do at least the basic analog signal conditioning (i.e. the parameters which probably will not be changed) prior to analog-digital conversion.
A similar question has been discussed recently:
http://forums.ni.com/ni/board/message?board.id=300&message.id=2119
