Pixel aspect for external matte

Hello,
i have clip from Arri Alexa that was sent to VFX department for some compositing. After VFX is finished i asked for matte to do some tweaking. And my problem is that Arri file is anamorphic 2880x2160 which is set in Clip Attributes to be CinemaScope in pixel aspect ratio to be displayed correctly... I want to do same thing to external matte which I got from VFX department but when I add it to media pool as matte to corresponding file, Clip Attributes option is grayed out and I can't get to change pixel aspect ratio for matte
matte is same resolution as Arri file 2880x2160 but because I can't change pixel aspect matte is useless
what would be solution for this?
i have clip from Arri Alexa that was sent to VFX department for some compositing. After VFX is finished i asked for matte to do some tweaking. And my problem is that Arri file is anamorphic 2880x2160 which is set in Clip Attributes to be CinemaScope in pixel aspect ratio to be displayed correctly... I want to do same thing to external matte which I got from VFX department but when I add it to media pool as matte to corresponding file, Clip Attributes option is grayed out and I can't get to change pixel aspect ratio for matte
matte is same resolution as Arri file 2880x2160 but because I can't change pixel aspect matte is useless

what would be solution for this?