If the value you need to set on the attribute is fixed, you could configure the importer to use a microflow. Just assign any other Excel column as input and return the hard coded value or constant.
If you need to actually process the data or assign a value based on other attributes before the object can be accepted, you could create another entity that only has the attributes for which Excel columns are available and use a microflow to process the imported data after the Excel importer has done its part.
When you call the excel import java-action, you need to input a filedocument and associate this one to your temporary object. After calling the java-action, you can easily retrieve the temp objects created by the excel importer and manipulate them.