How to install systemd-independent udisks2 and policykit before removing systemd from a Debian jessie/sid installation
Add additional repositorys to /etc/apt/sources.lst . From Devuan:
deb jessie main
deb sid main
depending on wether you have jessie or sid.
You also need packages from max2344, there are only packages for sid, but you can use them in jessie as well:
Reload your repositories by using synaptic or
# aptitude update
Install the following packages from the first repository:
udisks2 libudisks2-0 cgmanager libcgmanager0
Optional: To stop complaints about missing keys you can install devuan-keyring.
Optional: To give the packages from devuan a higher preference you can install devuan-baseconf.
Install the following packages from the second repository:
libpolkit-agent-1-0 libpolkit-backend-1-0 libpolkit-gobject-1-0 policykit-1
Now you can purge systemd while still having a working udisks2 and policykit.