Backlinking to domains ?

Hi

I have 20 domains that are doing nothing. What I want to ask is can I back link to them while they are parked in the hope of improving their PR ? I want to sell them individually when they are a minimal of PR 3

Can this be done ? or do i need to host them ?

I think most parking companies doesn't allow you to build backlinks to parked domains. Do confirm it or you might risk your current earnings.