It Makes a Difference to the Sheep

According to this piece in The Japan Times, they're up to all sorts of things:

Heilongjiang province, with its abundant farm land, is a popular destination. North Korean women marry farmers, filling the gap left by female infanticide and abortion prevalent in China. Others move to big cities. My favorite restaurant in Shanghai is owned and run by North Koreans.

Some move on through China and into Mongolia and Vietnam, either staying there or seeking to move on to South Korea. A planeload of such migrants recently arrived in Seoul, allegedly from Vietnam, although this was never officially confirmed.
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Some are beggars, some are petty or not-so-petty criminals. Handbag snatching and bank robbing by North Koreans is said to be increasing. Some of the criminals are rogue North Korean Army officers seeking to augment their poor pay.

Some criminals are part of organized-crime syndicates working with Chinese colleagues in drugs and prostitution. Others organize smuggling on a large scale, both ways, to feed the growing numbers of private markets in North Korea and to escape taxes in China (luxury cars and even earth-moving equipment from Japan are smuggled via North Korea into China).

North Koreans running restaurants in Shanghai? Who would've thought?

The article's author, David Wall, makes the common sense observation that there simply is no way the PRC authorities can track down, round up, and deport all of the North Korean illegals in China even if they wanted to:

There is no witch hunt to clear China of illegal North Korean migrants. There cannot be. The Chinese police simply cannot scour the vast areas of isolated mountains and forests looking for migrants or identify them in towns where there are hundreds of thousands of ethnic Koreans.

If he's correct (and he most surely is), North Koreans in China are there to stay for the foreseeable future.