Blowing in sidewall insulation with thin batts in there
Very hard to get any kind of quality with them in there. If they rip and drop while filling, even worse. Compressing the existing will negate it and adding another won't give you the R-value as if it wasn't there. If the old is faced, it should be to the warm in winter side if in a cold climate. Using dense-pack cellulose may work better, if done by a professional.....
If any ads are present below my answer, I do not condone/support/use the product or services listed, they are there without my consent.
WELCOME to the forums, if I forgot to say it!