Change Options for the AVR Programmer via Arduino IDE
Short answer: I'm afraid you can't do it with Arduino IDE 1.0.5.

Long answer:

First of all, be aware that -B20 does not specify the baud rate but the bitclock period (in us); this is specific to stk500v2 programmer.

Normally, enabling your programmer should only be a matter of adding it to the list of programmers known by Arduino IDE; that list can be found in hardware/arduino/programmers.txt. You would then append the following lines at the end of this file:

strangeprogrammer.name=Somehow Strange Programmer
strangeprogrammer.communication=serial
strangeprogrammer.protocol=stk500v2
strangeprogrammer.force=true
strangeprogrammer.speed=?????


However, the problem here is that Arduino IDE does not seem to be able to use flag -B (bitclock period) but only flag -b (baud rate) which value is set to whatever you will put to strangeprogrammer.speed. 

At least that's what I could find out by inspecting Arduino IDE source code: AvrdudeUploader.java never adds that flag to the avrdude command-line :-(

That means your options are:


Rebuild the Arduino IDE 1.0 on your own after modifying AvrdudeUploader.java to support -B flag; code should be quite easy (about 2 more lines of Java code).
Switch to Arduino IDE 1.5 and follow Federico's answer :-)
Use another IDE, something that deserves the IDE name. I use Eclipse with this Arduino plugin and it works fine for me. As a bonus, avrdude support seems better as you can see on the screenshot below: