Can I use diodes as part of a circuit to multiplex several analog signals?
Draw on paper.
 Post anywhere OR email to me and I will post. apptechnz@gmail.com. 

ANY diode in the signal path will affect accuracy badly.
 Probably error of about +/- 0.1V best case or 2% at 5V or 20% at 0.5V.   

If you must multiplex ADC use a "real" multiplexer IC to start.
 The cheap CD4051 (HEF4051 etc) 8:1 multiplexer will do well enough for playing. 

Pricing here - in stock from 52 cents US in 1's.

Datasheet here

eg see below. They use an external ADC but the method is identical for onboard ADC.