Create Custom Mask?

Hello.

I have recently seen some Lightroom presets showing lens flares and light leaks and such.
I wanted to create some for myself. And thought... the easiest would be to create a shape in Photoshop (maybe derived from a real lens flare), and then... somehow get that into lightroom.

I tried importing an image, creating a luminosity mask. But then there is no way to actually convert this into a standard / brush mask. Like a 'flatten masks' tool for example.

So - is there another way to copy, paste or import a custom mask into lightroom?

Or is the only way to do this, to actually repaint that mask with a brush in lightroom?

Thank you.