If the spindles are bolted to the axles you can add electric or surge brakes. If they are welded to the axle you can't add brakes.
and many other companies sell kits to add brakes to trailers and dollies. There are many different sizes available.
Here's a link to some kits available at e trailer:
Tow Dolly Electric Brake Kit | etrailer.com
Note on page 3 there are several kits that show different mounting hole patterns and styles. Here they show both 4 and 5 hole mounting patterns. Also on the 5th line from the bottom on the far right is a style that has studs welded to the backing plate rather than having the studs protruding from the spindle mounting plate.
If you're interested in adding them to an existing dolly I would call or e mail their technical experts with the brand of dolly you're interested in. They will be able to tell you what you need to do the job, and how much it will cost.