I have now added the [FP] constant to the tool. This change will be included in the build 59 patch 3.
Meanwhile, you can skip the "edit in Excel" step if you create a custom grid layout, with a
dynamic calculated column set to show the file path+filename "FP+F".
You can then export this additional column, already filled with the data you want, and, while importing, you just need to redirect this column to the field you want to use.
Note: You shouldn't export the field you are going to use, e.g. keywords, as final destination of this data, or you will have to unassign it while importing.