What do you want to use a download manager for? Is it for resuming, or for accelerating the download, or both? If it's the former, you could use
wget -c url/file
It's safe in Whonix, as everything is torified.
For the latter, I am not sure an accelerator would have much effect with Tor. Patrick could confirm, or disprove.
It's generally not a good idea to install other plugins than the default ones in Tor Browser, as they might compromise anonymity. And probably not a good idea too, to extend the profile for such use. Pushing the logic, if users are willing to take the risks doing so, they might as well disable AppArmor for the Tor Browser.