How to clump store-bought granola?
this might just work:

let's say there's about a half-cup (or just over 100 ml) of crumbs 


preheat your oven to 200F (about 90C) with the door closed 
take a paper towel and tear it in half 
get one half completely wet with water, but squeeze it out enough so it's not dripping 
put it in the bag and shake-shake-shake.  this gets a little water more-or-less evenly on the surfaces of the crumbs 
remove the wet paper towel and shake the clinging crumbs back into the bag 
dump it all on the dry half of the paper towel and roll it up into a sausage shape 
set it in the oven on middle of a rack in about the middle of the oven 
close the oven door and turn off the oven 
wait at least 4 hours 
voila, 1 granola roll, i hope