Is it save to install packages from raspbian?

I want to try to install mpdscribble_0.22-5_armhf form

because is not in a official Volumio repository.

Is it save?

Not really…
In short, on the flashed version, your upgrades to the rootfs will cause no harm and will work.
And as long as you stick to that version, all will remain fine.
But there is no guarantee that this is still the case if you run a Volumio update as it replaces the rootfs.
Because your changes were persisted by the overlayfs filesystem, they may not necessarily match the rootfs after an update.
There has not been a lot of testing in that area.
My advice: don’t do it unless you you’re ok reflashing the device whenever a new Volumio version arrives you’re interested in.

I would say that mpdscribble is fine, as it won’t touch anything on the core system…