YES! you can add Arabic text to affinity Photo or Affinity designer. This method used in photoshop as well when didn’t support Arabic text.
You can follow this video https://youtu.be/awEwjgMaokI
Video steps given bellow:
1st. Download arabic font. I prefer https://arabicfonts.net
2nd. Imported to Affinity photo or Affinity designer by going to settings—> fonts—> choose cloud icon —> import your downloaded Arabic font.
3rd. Go to http://www.arabic-keyboard.org/photoshop-arabic/ Site and type your arabic text and then you have copy option to copy it. Then go to affinity normally paste it in. Make sure to use that Arabic font in affinity app