Remember that much of the reason North Korea is the way it is currently, is due to active suppression of many types of economic activity that would otherwise be happening naturally. There's a lot of economic "potential energy" in NK waiting to be unleashed — e.g. many NK entrepreneurs currently doing grey-market activity while hiding from the regime, who'd love to become reputable businesses and market their services in the open; many NK workers who'd love to take jobs in the inevitable call centers SK would build there (NK would instantly become to SK as the Philippines is to the US — a country full of native speakers of your language, that you can put in front of phones); etc.

Also, probably one of the first things to happen, as soon as it was allowed to happen, would be that North Korean land-owners would sell their land to South Korean farming conglomerates (at ridiculously low prices); those farming conglomerates would then come in and apply modern agriculture practices, and be growing 100x more food within the year than the NK farmers were able to grow on the same land area.

Yes, by default those SK agro-businesses would be growing for export ("extraction economy"), because SK could pay far more than NK could for the produce. But the NK government could just tax that economic activity — it's not an SK business, after all, but an NK-incorporated subsidiary of an SK business, subject to NK laws. The NK government could then use the tax revenue to turn around and buy the food grown by those businesses, to distribute it in social welfare programs. (Or they could just levy taxes directly in the form of produce, ala historical agrarian-economy taxation systems.)

(Interestingly, there should also be at least some things you can grow cheaply in NK but not SK, due to the latitude difference. SK is parallel to Spain/Greece; while NK is parallel to France/Northern Italy. Agro-businesses love expanding into these alternative growing regions to expand their TAM.)

All these positive things would happen quickly and easily (i.e. within a 3-year period), just by converting North Korea into a capitalist-market-economy + social-welfare state and enabling free trade between NK and SK (and between NK and everywhere else, too.)