No,Castille would probably object and prevent any annexation attempt as per\OTL under Ferdinand II, if there were a personal union between the two though then yes but it would occur later and there is the Treaty of Tordesillas to consider, Does Aragon get a zone as well and where would it be. Otherwise they would be in the same boat as the French, English and Dutch in intruding on Spanish (Castillian in this case) and Portuguese claims. Since \india and East Asia is where the money is at Trading posts in Africa along with Portugal is the way to go. The Portuguese of course have a head start and how they would reach agreement is unknown, but the Papacy will no doubt have some role.
say Castille in the West as per OTL but in a line further west since there is an extra player. and Portugal and Aragon in the East, based on who gets there first, though the pickins will be slim unless they act fast and its hard to a concentrated motivation given their need to concentrate on the diverse maritime possessions in the Med.
Okay,scratch that...they willhave to have part of the Castillian zone say a line drawn in the south (tropic of Capricorn say). but of course how do they get there... alternatively a line further west of the Tordesillas line ( Say 65-68 degrees West). the latter would give them a bit of the Caribbean and the n coast of S. America and the Rio de la Plata. Is there really any reason to go establishing a presence, no, but they wouldn't know that until they actually attempted.