The reason any government would want to build a foreign currency reserve, as I've stated in another post, is to help mitigate a currency crisis. In the case the Renminbi has a large drop in value, they would start pumping dollars in and pumping Renminbi out. By decreasing the supply of Renminbi, and giving folks a supposedly more stable currency to hold onto, they can fix a currency problem.
If the Bank of China were to spend US dollars domestically, it would be exactly equivalent to printing Renminbi and spending it. It would be just like a stimulus plan. There's nothing different. Remember, Bank of China must print all the Renminbi necessary to keep the exchange rate at the peg. So watch this:
1. Chinese government spends 100 million USD for public works project
2. Businesses receive 100 million USD, want to convert it to CNY, bring it to USD-CNY exchange market
3. Government prints CNY to soak up 100 million USD
We are right back to where we started, except now there is more CNY in circulation.
Now if we relax the assumption in step 2 that businesses want to convert to USD upfront, we still have the same problem, because it just delays the conversion. While USD is in circulation, there will be some converted to CNY at every transaction (for wages and taxes and things like that).
If the government wanted to enact a stimulus plan, and I think they already did, then they should just do it in CNY.
Note: China does not maintain a strict CNY-USD peg. CNY is pegged to a basket of currency.