Currently it's not possible to paste into filter layer, filter mask or transparency mask. It's of course possible to convert paint layer into those. However this solution is limited. When filter masks or filter layers are heavily used and filter parameters are not changed only mask content, it requires recreating parameters and reapplying filters again and again that can be really tedious and time consuming. Solution is to allow pasting into filter layers, filter masks and transparency masks similar to this PS feature: http://blogs.adobe.com/jkost/2015/04/how-to-paste-into-a-layer-mask-in-photoshop.html
Ran into this issue. I was following the "Split Alpha" documentation here: https://docs.krita.org/en/reference_manual/layers_and_masks/split_alpha.html And wanted to copy data from another image/layer to place into the Alpha channel. This was for a game asset which the feature is promoted as being useful for catering to, but as I was wanting to merge external data rather than create/edit the Alpha channel, this doesn't seem possible? Earlier I wanted to shift color channels around too or work on those individually, and the process for doing such is also quite bad with Krita, Photoshop handles this scenario really well.
@Brennan Kinney - with questions is something is possible etc. and a specific workflow you're not sure about it's much better to go to user support forums and websites like krita-artists.org or reddit.com/r/krita or https://forum.kde.org/viewforum.php?f=136 . In your case, most probably you can just convert the layer into the transparency mask and put under the correct layer. You can leave it like that, but if you want to get rid of the mask or have just one mask for transparency: if you want to *replace* the transparency that was there before, use "Write as Alpha", but if you just want to make parts that were previously opaque more transparent, just use "Flatten Layer".