OTL, Angola was practically a colony of Brazil until Brazil's independence. Brazil would have much liked to make it official after independence, but quite realistically knew they weren't in any position to attain it.
the easiest way is to have Napoleon successfully take over Portugal, either by sticking to the original plan, or having the Spanish takeover be successful. From there, the crown is permanently settled in Brazil, and Angola is part of the empire. Or alternately, enough time passes before portugal regains independence from France/Spain and Brazil is strong enough to dictate terms at their own independence. Perhaps Portugal decides to go republic, and the rest of the empire sticks with the crown.