yes collapsing a multi-vsys into a single vsys configuration is a use case for Expedition.
Think of Expedition as a PanOS configuration editor. After importing the 2 configurations you can then use Expedition to move objects from one configuration to the other. In your use case you will want to set the single VSYS config as your base configuration (where you are moving configurations to).
Prior to starting the recommendations is to have a design in mind. For example, when you collapse the multi-vsys config into a single vsys, you will want to consider design options to include:
-if both vsys are layer-3, will you be collapsing to a single VR or use multi-VR's in the single vsys config.
-will you be consolidating VLAN's onto a single trunk
-will some interfaces be combined into same zones
there are many other design considerations, but its recommended to have a design goal prior to starting.
