Can you give an example sentence for the use you mean? Because if you meant “as well as = and also”, then I’m afraid you are incorrect. For example “as well as” in this sentence cannot be translated as außerdem als: “The room was furnished with a couch, a TV, as well as a couple of cabinets.” You could use und außerdem or und darüber hinaus. Or alternatively you could say sowie, which is basically a slightly fancier replacement for und when you list things.
At least as far as I can see, außerdem als is quite literally “also as, as … too, as … as well”. It’s not a phrase which belongs together but an adverb and a conjunction which happen to occur after one another in some particular sentences such as ours above. There is nothing more special about außerdem als than there is about außerdem in …, außerdem bei … etc. At least not in any sentence I can think of right now.
"Das Dach ist außerdem zu alt" has recommended translation "Furthermore, the roof is too old." But for the present sentence, "furthermore" is rejected for "außerdem", though it works just as well in English here as in the sentence about the roof. I wish Duo were more consistent!
I don't think "otherwise" means exactly what you think it means. "Außerdem" means things like "furthermore", "moreover", "in addition", etc. "Otherwise" translates to words like "andernfalls" and "sonst", so using "otherwise" instead could mean for example that he if he weren't too busy being a composer, he would be an author.
If you talk about who somebody is (function, profession, nationality etc), you usually don’t need the article in German. Remember how you also normally say Er ist Autor rather than Er ist ein Autor. The latter sentence is also possible, but would sound like there is a particular group of authors, and he is a member of that group, rather than like we’re talking about his profession.
Der Schwede (without -n in the nominative) ;)
But yes, to say “he’s a Swede” (talking about his nationality) we say Er ist Schwede. Er ist ein Schwede is also possible, but that would mean that you have either a group of Swedes or the collective of all Swedes in mind, and you are expressing that he is one among that group/an example of that group.
This is also why JFK’s famous sentence Ich bin ein Berliner is actually correct – because he was not saying that he is of Berlin origin, but rather he counts himself among the collective of Berlin citizens (for a more in-depth discussion, see this video).
In fact that is the only interpretation I can imagine. I can’t think of any sentence where außerdem refers to the subject; it pretty much always refers either to the action as a whole or to the object. So in this case it could mean “besides doing other things, he also works as an author”, or if it refers only to the object “besides other positions, he also works as an author”.