Okay to blow cellulose into existing wall cavities that have fiberglass?
Just my opinion, don't bother.
You have insulation in the walls. That's good but your greatest heat loss is still the attic.
No matter what they do, with 2x4 walls, at best you can only get about R-13 in there. Then there is the problem of compressing the fiberglass. You don't want to compact the insulation, it is supposed to be "loose." It's the air pockets that actually do the insulating.
I doubt they'd actually manage to get the entire bay filled.
You probably already have 2" batts at a minimum, about an R-7. You may even have R-11 in there already. Without opening a wall, you won't know.
If there is NO insulation, then I'd go for it.
With the labor (and patching) costs, and some existing insulation in the walls I'd spend the money on the attic.