If I have a regulated 5v supply how can I power a micro-usb b device?
Should be no problem to just wire to it, just remember the pinout (don't worry about type A or type B just leave pin 4 unconnected). Also you should technically be limiting the power on the USB power lines to 500mA to comply with USB standards. The RasPi, however, may need more than 500mA of power and be unstable if powered only from USB limited at 500mA.