The answer is yes, and not that hard, but not immediate, and not for the faint of heart. However, it takes less time than it took me to write up this post. I tried to be thorough, so bear with me.
There is, as far as I know, no way to do this directly in the application GUI.
I was able to achieve it by a rather roundabout but effective way.
And, as often happens, it involves xml.
I will give you a recipe that works, but it needs to be applied very precisely.
Suppose that, as you describe, you already have that Prism effect all done.
There are two easy preparatory steps:
0a) duplicate that Prism effect, exactly as it is - (this second copy of the effect will be then "converted" to Blur, in a subsequent step, via xml):
Select the clip that has the effect, press Command-C to Copy;
then press Command-Shift-V to Paste Attributes: disable all effects except Prism.
This effectively makes it so you have two identical instances of the Prism effect applied, including the masks - you can check that in Inspector
0b) add the Blur effect, don't bother with any masks at the moment - this one Blur will be deleted in the end, and it is just to make sure that a reference to Blur appears in the xml
1) Export xml of the project - select the project, or click on the timeline to make it active, and File->Export XML...
2) (this is the more delicate step)
2a) Open the xml in a text editor like TextMate or BBEdit (TextEdit should work too, but make sure to always keep as plain text).
In recent versions, the actual file info.fcpxml and is inside a bundle with extension .fcpxmld.
You will first need to control-click and choose Show Package Contents to see the info.xml
2b) Search for "Prism" and note the number associated, e.g. "r6"; same for "Blur", suppose it's "r7"
2c) Search for "<filter-video-mask>"
You will see the description of the mask parameters (and recognize the names you have given them, such as "Anthony Mask", etc)
and subsequently lines like:
</mask-shape>
<filter-video ref="r6" name="Prism"/>
In the second instance where you see those lines (this is why you needed to duplicate the effect), change it to
</mask-shape>
<filter-video ref="r7" name="Blur"/>
(of course, replacing r6 and r7 by whichever identifiers appear in your xml file - these are just for example)
3) Save the file
Note: if you are unable to save directly because the info.fcpxml is inside a bundle, save it temporarily to the desktop, then use Finder to replace the info.fcpxml file inside the bundle with this new version
4) Double-click the now edited fcpxmld bundle to open it. I suggest that you choose to open it in a new test library just to make sure everything is ok; once you are satisfied that it is, copy this new project to your working library.
