MIDIKEY2KEY

My ultimate objective is to take my Korg NanoPad2 and using the buttons to generate notes / chords, send these messages to my Korg Oasys. Bascially using the NanoPad2 as a Midi Controller into the Oasys.

The NanoPad2 is powered by USB. Therefore it appears it must be connected to a computer.

So I have the Korg NanoPad2 connected to my computer
It is recognized by MidiKey2Key (MK2K) when I select the device for Midi In and can see notes on/off messages in the Midi Input Log
This verifies that the NanoPad2 is communicating to the computer and to the MK2K s/w

I have an Edirol USB to Midi converter
I select this device for both Midi In and Out with MK2K
The Midi Out is connected to the Oasys
I use the editing software of MK2K and create a Note On / Off Action
When I test this action, a note sounds on the Oasys ... as expected

I then Select Midi In as the Nanopad2 and Midi Out as the Edirol USB
I perform the same test action ... nothing / no sounds out of the Oasys

So my first objective is getting the 2 USB devices to work with MK2K

Any ideas?

My next objective would be to be able to press a button on the NanoPad2, receive the Midi message on the MK2K, and forward the message to the Oasys (thru the Edirol USB interface)

I do not know, if I understand You correctly.
Try to turn on "mirror MIDI" in the options.

Can I mirror some but not all commands? I mean e.g. a mirror for the fader and a some exceptions for the buttons?

No. All or nothing.