I used Photoshop to add an alpha channel for the prop. What I do is make my prop blur alpha texture, and then add it to my prop PSD file in Photoshop as an alpha channel.
In the layers box, there is a tab next to "Layers" which says "Channels".
There, you would normally see three channels, assuming the file lacks an alpha channel. If it does, "Alpha" should be present (you can add it). This is where your alpha channel texture goes. I did the same thing for my prop blur.

After which, I saved my PSD (preferably the same name as your prop texture), and used the ACES ImageTool to convert it to a format FSX can recognize (e.g. DDS with DXT5 compression).
I used
this FSDeveloper wiki article for help, and it paid off.