Lets say I have already registered in a website and created a user id and password but, now I want to link my existing account to a OpenID account like Google, so that I don't have to remember password for multiple account, is there a option for this? e.g. www.technologyreview.com
This is going to depend totally on the website and how they have implemented their registration process and whether they recognise OpenID or not.
If they do recognise OpenID it should be possible to link your OpenID to your account - but unless you have a specific site in mind that's all I can say at the moment. As for the site you mention, I've not checked yet but it should be pretty clear from your account page whether you can add an OpenID or not.