Installing wodpress Multi-DB or Hyperdb

I want to install Multi-DB or Hyperdb... our multisite will host a lot of blogs it not confirmed yet how many... can I add the multi db later... I mean when I need it... suppose I have 1000 blogs then I want to install it... or is there way I can add database when I need one... suppose I have 2 db and I need another one then I add another one. I use cPanel I can not create database from sql can you help me about it? I have issue with back up... I can export it but how can i import it later?