Adding Robin Hood to Greyhawk is easy.  Although it can be considered a Heresy.  The group has to be added where there is a forest.  And probably while a Crusade is going on.  It's also a good idea to pick up either GURPS Classic Robin Hood from Warehouse 23, or the Campaign Classics: Robin Hood for Rolemaster and Fantasy Hero.  The last is available second hand on Ebay.  You might also find Howard Pyle's work on Robin Hood a good read. 

If you want to delve deeper, there is a bibliography at the back of GURPS Classic Robin Hood that provides you a good starting point.

The following nations are good for adding Robin Hood.

Furyondy -- the nation of Furyondy currently (as of 591 CY) started a Crusade against the Empire of Iuz.  Belvor IV could be like King Richard I of England.  Unlike the Crusades of our world (they are complicated in History), Furyondy's Crusade against Iuz is a moral Crusade against a real evil.

 The Prince John character could be a brother or baron looking to expand his power.  Sir Guy of Gisbourne could be a knight that stayed behind to help guard Furyondy. Same with the Sheriff of Nottingham (just choose a Furyondy city or province that would work well)

Robin Hood could be based in the Vesve Forest.  He could be a highway man robbing from the rich to give to the poor.

The Duchy of Ulek -- The Duchy of Ulek has the Silverwood within it's borders.  A perfect place to place Robin Hood and his merry band.  

The duke of Ulek could be away for some reason, and a treacherous brother could fill what is needed for Prince John. The Sheriff of Nottingham becomes a Sheriff of what ever county in the Duchy of Ulek you need him to be. And Sir Guy would be part of a county of the Duchy.

The Duchy of Ulek was where I set Robin Hood for my own Robin Hood campaign.

There are other nations where you can set Robin Hood.  The Great Kingdom of Aerdy can be a good fit.  But the two places above are my suggestions for setting a Robin Campaign.  Reading through the Greyhawk Campaign Setting (circa 1983), the Living Greyhawk Gazetteer (circa 2001), or Greyhawk: the Adventure Begins (1998) and reading through various sources for Robin Hood can give you more ideas on how to fit Robin Hood within Greyhawk.