There are ATMs where you can get extra cash only in a few places on certain trekking routes, such as Jomsom in the Annapurna Region and Namche Bazaar in the Everest Region. However, it's best not to rely on these ATMs since they tend to run out of money or are sometimes out of service due to network connection problems. If you are desperate, there are lodges and shops that will change major currencies at a poor rate along the trail. There are also a few places on major routes (Annapurna and Everest Base Camp) that give cash advances on credit cards for a fee. If you are planning to take out money from an ATM in Kathmandu for your trek, make sure you check with your bank to find out how much money your bank allows you to withdraw in a day. Most Nepal ATMs only allow withdrawals of no more than 35,000 NPR per transaction (many less than that), so you may also need to make multiple transactions to reach your bank's limit.
More about changing money and ATMs in Nepal can be found on our blog:
ATMs and Changing Money in Nepal