I guess it goes without saying, but knowing the lawyer-y types around, I speak on everyone's behalf: "GBATemp does not condone illegal activity."
Now, here's a bit of a thesis on getting free DLC's.
Certain cards can be used to confer value to one's online gaming account, equal to the agreed-upon purchase price of the card.
Once the gaming account was sufficiently funded, said DLC would be purchasable.
There was a time when certain cards could be keygen'd and activated with leaked software, likely from the POS of a GameStop or something.
Those times are long-dead with newer activation measures, security protocols on-site, heavy logs to audit, and authorization revocation abilities.
And, employees at companies capable of authorizing these codes face civil and criminal liability for fraudulently activating stolen cards. It's an effective legal deterrent.
That's looking at it from the POS/POP side.
The 3DS is a very secure system from the looks of it, as Nintendo has learned from their previous consoles that historically, unchecked system-level homebrew can, and usually does lead to piracy.
There are efforts to understand the inner workings of the 3DS, as, being a very capable handheld system, one could run more powerful homebrew applications and games on it, and customize one's 3DS software to their liking.
However, due to the first point, it may take a large effort to make it feasible for the typical 3DS user to activate DLC which has not been purchased.
That's from the 3DS side.
Nintendo's servers are tightly-woven loadbalancing datacenters spanning diverse geographical locations. Compromising the servers from physical space is a no-go.
Nintendo's web team are only given enough permission to change the templates and layout for their sites, I'm sure. Getting the codes from them, too, is a no-go.
Compromising the servers from internet-space would only afford you some Apache configurations and site files/templates.
This has actually happened, and I won't tell you where to look, but there is a mirror of these configuration files posted online.
They are boring.
The worst you could do, is spoof Nintendo's website on your own server. Yawn.
The activation system is likely accessed through the website via an offsite database system, with its own protections against unauthorized use, and only accessible from the web server hosting the page.
Permissions to do anything other than what is directly necessary through the UI on the site, would be denied.
Now, there is administrative power that can access these behind-the-scenes things, and there are several staging servers which may hold debug/testing software, but they would more than likely require access to be signed off by someone who holds the keys to access them.
Whoever holds the keys at Nintendo are a high-echelon team of decision-making people just short of, and possibly including, Iwata himself.
So getting free eShop codes from the servers and the people that run them, is a no-go.
However, and this is quite the big "
secret", but if you work for Nintendo, among others, you get remuneration crediting your time and labor spent, that can eventually be used to acquire DLC titles!
These "credits" usually differ by country, and are converted between them at fluid fractional rates correlating to the worth and wealth of the nation and its people.
They also can be converted into goods and services, or in this case, a currency/commodity-backed private fixed fiat currency in an input-only single-purpose account owned by a large media company, with the sole intent and purpose of increasing the revenue of the host company.
The economist in me screams bloody murder at the slightest intimation that people actually do this, but the ends justify the means. Capitalism: it works, bitches.
The point is, said "credits", however you acquire them, can be easily converted. In this case, to Nintendo eShop points.
These points can then be used straight-up to acquire the DLC's as you wish!
The secret to getting free DLC's? Acquire these "credits" through the means necessary and convert them at your earliest convenience to the eShop points.