Well first of all, why would America take Spain? And even if it took Spain i find it very implausible for them to keep Spain as a part of the Unites States. A client state maybe, but not part of the country. Then you have other problems, like the Spaniards willingness to be controlled by a country on the other side of the Antlantic, not to mention the huge logistic and economical problems of controlling such a huge country on the other side of the ocean. And why would taking Spain make the Americans join the Central Powers?
But if something like that does happen, it would have some pretty big consequences. There migth be more immigration from Spain to America. The minorities in Spain like the Basque and the Catalan might start demanding more autonomy. There could be a huge resentment against the Americans, and i find it impossible that the USA would be able to keep Spain for that long (till WW1). One scenario that i could see is the Americans promising more autonomy to the Catalans and Basques, which in turn makes them more "pro-American" and the Spaniards more "anti-American". This would have some very long term effects in the future, like Spain breaking up, becoming independent, but without Catalonia and the Basque country, which could remain part of the USA for a bit longer but demand independence after some time.