The voltage that you get from this configuration will be the sum of the mains voltage, and the normal secondary voltage. Also please keep in mind that because this turns your transformer into an autotransformer, that there will be no mains isolation. Additionally, I recommend that you place something resistive in series with the mains leads to limit current
Hope this helps you, play safe. :)
Troubleshooting: If you wire everything up and get the exact opposite of what you should (the secondary voltage is subtracted from the primary voltage) then just reverse the secondary wires.
Safety: Configuring a transformer in this manner can produce dangerous voltages, with little to limit the current. This
combination can be very dangerous. Always use proper safety precautions, and above all: common sense.