Advice on a existing perlite texture

I am a taper/painter journeyman. A gentleman from work came to me and asked if i could fix some hollow seems on his cathedral ceiling in his home. I went to his house and come to find out his ceiling has a perlite/bumpy texture. Fixing his seems isnt my issue, its how to blend the existing to the new. All the seems need to be recoated on his ceiling, so with that said whats an easy way to blend it all in? He resides there so a hopper is out, and i'm not or he is paying for one. Is there a particular way to blend it in with a paintroller? If so whats the procedure? how big a roller, what type of perlite, paint, or joint compound do i need? I will take any advice I can get, he is a friend of mine, but I'm not doing this for free!!
Yours Truly
A friend in need of advice