LONDON—The advent of digital currencies in emerging markets could spark “cryptoization” of local economies, potentially undermining exchange and capital controls and upsetting financial stability, the International Monetary Fund said on Friday. Bitcoin and its kin have in the last year soared in price and popularity, with emerging and developing market economies such as Vietnam, India, and Pakistan seeing rapid growth in some measures of adoption, according to U.S. blockchain researcher Chainalysis. Cryptocurrencies offer, in theory, a cheaper and quicker way of sending money across borders. Backers say digital tokens such as stablecoins could also help protect savings from high inflation or fluctuations in local currencies. In September, El Salvador became the first country in the world to adopt bitcoin as legal tender, with backers tipping the experiment to lower costs for billions of dollars of remittances sent to the Central American nation. The IMF said that unsound macroeconomic policies and inefficient …