can you put pain boyz on bikes, and get them additional armor like the rest of the nobz in a nobz unit?
There is no point in buying eavy armour for any Nob/Painboy on a Warbike; the Warbike gives the rider a 4+ armour save, giving them eavy armour would just give the same save at a waste of 5pts per model.
It should also be pointed out that the upgrade to Nob Bikers specifies the "entire mob may be upgraded", where as the the option for Eavy Armour is "Any Nob may take". Two very different meanings.