On import, only the child resources have User Field values. I want to know the total Volume for the parent resource (eg. "Floor (7)")

You may create a new User Field for total volume
1. Create a Resource User Field of type Number and set it to Calculate.
 
2. For the Formula use:SUM(RESOURCE.UFV("User Field Name"))   - For each Resource, this will sum the value of the specified User Field for all child resources nested underneath (it will only assign a value to parent resources whose child resources have a value for the specified User Field being summed

 3. Then from the Project Controls ribbon, select Recalculate Values and make sure that the "only leaf objects" option is unchecked.

 4. Right click in the header of the Resources table and use Customise Columns to add the new User Field to the table and see where values have been calculated for this UF. You may also see the values in Resource Properties>User Fields for parent resources where the value was assigned