I'm in the process of renovation of a 1945 house. Originally built with a flat roof, tar and gravel. At some point a gable roof was constructed over the original flat roof. Space between the ceilings and flat roof is 10 inches, there is insulation in that space ,about 2 inches of paper backed fiberglass. I would like to add more insulation with out removing the existing ceilings. My plan was to blow in cellulose on top of the tar/gravel roof. Does this make sense.
I'd blow cellulose into the 10' space, from above. I'd worry about moisture and rodent problems with cellulose between the old tar & gravel and the rafters. The tar & gravel makes for a moisture barrier you don't really want.
Check your venting in this attic space, and consider a humidity controlled fan. You probably have K&T wiring in the 10' space, if so see https://diy.stackexchange.com/a/20279/5960