How can I add the rel = "canonical" tag to wordpress posts?

I am not sure if this is the right place to post this question, if it has been asked then please point me in the right direction. I posted this same question on Goolge webmaster forums and they told me to come here:

Hi

I have a business where I provide a service across the UK.

There are many different services that all could fall into the same catergory, because of this I have a seperate site for each service I provide.

Eventuallly if a client wants to book with us they get directed to the main website which is the name of the business.

On each of my service sites I post reviews abour particular products which are relevant to that service.

What I want to do is also post these reviews on my main website.

I want to be able to show Google which website should take the credit for the original post and after reading about the rel canonical cross domain tag I have tried to find on the net where I should be adding this to my posts?

I use wordpress for all my websites, can I add this tag where the 301 redirect section is on each post or do I have to go to my server to add it individually to each post?