Hi gris,
I don't want to over think your question, but is this a custom drum sound you've made using the the Melas Motif Waveform Editor or are you just talking about attempting to edit a preset snare Waveform with the Yamaha MOX6/MOXF8 Editor?

Preset Waveforms cannot be edited on the Keybank level inside any of the Yamaha synthesizers. A Waveform is a collection of individual samples. There can be a maximum of 256 individual samples in a Waveform. A Sample is defined by a Note Range and a Velocity Range. Why 256? There are 128 Keys (C-2 thru G8) each can hold two mono or one stereo Sample. Or you could stack samples vertically by Velocity Limits... where the velocity determines which one is sounding. Most configurations are a combination of horizontal and vertical mapping.

For example, a Normal instrument Waveform could contain scores of Samples... the CF3 acoustic grand Waveform (a soft, a medium, and a hard strike) - each set of Samples for each layer. When you affect some kind of Editing change to the Waveform it applies to all samples contained in that Waveform.

A Drum instrument Waveform could contain multiple samples as well... but the samples are stacked vertically (Velocity) because the Drum instrument typically occupies a single key. So you could stack the multiple samples vertically to be triggered by different velocities. Many of the Snares, for example, are five way velocity switches... when you edit the Waveform you are applying the edit to the Waveform as a whole. (Editing individual samples takes place in the sampler)

In order to edit an individual Sample Keybank, you would need either the Motif XF (hardware) - it has a sampler built-in, or as my original question, the Melas Motif Waveform Editor. The XF hardware has a sampler (that is where you can construct your velocity layers, and you can tune, shape, and otherwise mangle, to a dizzying degree, your individual layer.

What you can do to create your own layers in MOXF is to use multiple Keys (multiple Waveforms) to construct your sound. Use multiple keys to create the snare sound, you can velocity limit each Key in your Drum Kit so each responds to the data as you require. You have 73 Keys in your Kit, (more than enough). Say you want to create a five way snare of your own... copy the snare MIDI data so that they all have Note-on, but restrict the velocity per Key, as you desire.

Of course, if you were talking about the Melas Waveform Editor, you can just select the Keybank and edit it individually... the Melas Editor will allow you to Output data in a file format you can load to your MOXF.

I was in fact using the yamaha editor, thanks for explaining about keybanks, I think I understand now, I also have the melas wave editor wich I will use next time.
Cheers