Thanks for your response Chris. I wish it were that simple. Here is the procedure that I have settled on, although there is at least one other method that works for me.
To insert a mask from an Adjustment Layer into a Filter Mask of a Smart Object do the following:
1) Activate the Adjustment Layer (containing the source mask) by clicking on it
2) Switch to Channels
3) Ctrl Click on the alpha channel to load the mask as a selection
4) Swtich back to Layers and activate the Smart Object layer
5) Apply your filter of choice
6) As you do #5, the mask from the Adjustment Layer is applied and the selection is turned off
I appear to have double posted. My apologies.
Regards,
Dennis