Hi Marc,
It would be possible to do something to the same effect as purchase invoice approval.
This can be achieved by editing employee permissions, restricting them to only be able to create purchase orders. Purchase orders can then be reviewed and processed into bills by a senior manager.
You can alter user permissions by heading to the building silhouette in the top right corner of the screen, then selecting "manage users" from the drop-down menu. From this screen, you are able to specify the individual permission for each user on the system.
I hope this helps, Theo