In theory, yes. Without extra parts it may be possible to power the 5V trinket from a 3v3 supply by bypassing the regulator and powering it with 3v3 directly through the 5V pin. But you'd be playing with fire.
Plugging in the USB port while this is set up would be bad news and potentially result in crispy fried Pi.
This would also mean you couldn't use the Trinket's onboard regulator to produce a decent operating current, so you would be limited in what you could run from it.
Alternatively you could set up a logic level shifter ( http://shop.pimoroni.com/products/adafruit-4-channel-i2c-safe-bi-directional-logic-level-converter ) and run your MOSI, MISO, CLK and RESET signals through that.