[Dacilndak]

Well, you could do something like

if (joystick1.GetZ() < 1.0 && joystick1.GetZ() >= 0.33)
{
	// tank drive
}
else if (joystick1.GetZ() < 0.33 && joystick1.GetZ() >= -0.33)
{
	// arcade drive
}
else // if (joystick1.GetZ() < -0.33 && joystick1.GetZ() >= -1.0)
{
	// holonomic drive (?) ... whatever else
}

Ideally, then, in the middle of driving you could turn the Z wheel (towards the bottom of the joystick closest to you, like davidthefat said), thus switching drive modes.

I wouldn't recommend using buttons to switch drive modes, as the change in speed can be sudden and you could tear up your drivetrain if the driver isn't careful and/or you aren't ramping your joystick inputs.