Switching positive and negative voltage with npn transistor (or multiple npn transistors)
Use an optically isolated AC MOS relay or a mechanical relay. 

Eg. 

If you are really insistent on using NPN transistors you could put the transistor inside a bridge rectifier and power the base using a DC-DC converter. It would have 2 diodes drop, plus a bit, however. 

Thyristors are another possibility but they don't turn off until the current drops below a certain level, due to external reasons (such as the source being AC). 



Based on your additional information you could consider a divider from +/-200V to 1.5V and a programmable gain amplifier to step the 100/20/5 up by a factor of 2/10/40. That takes an op-amp, a low voltage analog multiplexer and a few resistors.