Conditionally using Validation drop down lists

[mmohon]

Based on entry of Item A, I want Item B pulled from a list. Sometimes Item B might not appear in that list, so I need to use a validation list to look it up.

Is there any way to conditionally use a validation drop down list?

=Iferror(vlookup(ItemA, inList, 2,false), validation list drop down)

I will put a macro in that pushes the chosen result to the look up list, so next time it will be in the list.