I miss that feature so much in Substance Designer .  Asked them for decade to do it .  So we could import a plane and squeeze it for some 256x2048 texture preview  without searching through hard drive.
And also thanks  for letting drop a packed texture and use a selected channel as roughness, metal  etc without any extra tricks 
I bought Marmoset just because of those things .  Hope to find more of a kind .  I am so tired of Allegorithmic products.  They seam just doesn't understand what simple and easy to use tools are.