Could the Stepstones be successfully and permanently added Westeros? What would it take?

I’ve had this one asked a bunch, so let’s get this one knocked out.

Permanently? Probably not, at the end of the day the Free Cities are not going to want their powerful western neighbor that close to their business, but do you need it to be permanent? England controlled Calais for almost as long as the Targaryen dynasty held power in Westeros.  

Successfully? Well, you’re going to need:

• a significant force of soldiers to act as marines and capture all potential ports so that you don’t have a situation similar to Marcus Antonius Creticus’ campaign against the pirates, where they just move shop to a different island and make you play whackamole. 
• a beefed up royal navy to sweep the Narrow Sea like a broom, driving the pirates before you and preventing them from doubling back or playing hit and run. 
• a commander with both ability and undisputed authority, because there’s going to be a lot of coordinating between army and navy and different regions and you want a clear chain of command to ensure that plans get carried out effectively. 
• really, really adept diplomacy. What you don’t want is the Three Daughters, Volantis, or Braavos getting involved because they think this is an act of aggression close to their home territories, because they could easily turn a doable if difficult campaign into a total quagmire. So promises are going have to be made and goodies traded to keep them sweet – you definitely want to make clear that tolls will be kept stable and low, that all nations will have free access to the Narrow Sea, etc. In fact, it might be worthwhile giving up an island to each of the Free Cities to keep them happy and avoid the impression of a monopoly. 
• That being said, build a lot of strong naval bases across the Stepstones so that taking the choicest islands back would be hard as hell, and so that any such attempt would end with a strong naval squadron and a hard-hitting force of Westerosi knights and bowmen knocking at the gates of the city that messed with you within a fortnight. 

In other words, you need to pull a Pompey Magnus and then don’t fuck up the peace.