Yes you can. But it is NOT worth to . SINUMERIK 810D is a "small" or compact version of 840D.

one of the advantages for 840D/810D is to control digital drive 

You don't mention what kind of CNC system (old one ) do you have and what kind of servo drive. if you really want to some upgrade post a new topic, I will give you suggestion 

I post here the Siemens manual cover for ANA control as your REF.  

asjalal

I have 3 axis cnc machine.

I want to know if it's possible for me to run it by 810 d with 3 analog servo drive??