i don't think its possible,
in order to import an Excel file into filemaker, the excel formal has to be the same as the filemaker table
Are the different variables in different columns in excel or the same column?
Usually this kind of "transformation" is handled by importing the data into a temp table then processing the records via a script to put them into the data table you want.
If they are in the same column one way to address the issue would be to add an indicator column to your spreadsheet and import that column as well. (ie add a column called Var_Names then V-1-2-3 or V-4-5-6 etc) then you can import the excel into a temporary new table and then process those records via a script creating records in a ID_VariableS table(the one you way said wanted to have it). You can use some case calculations to set the VariableName based on the numbers in the different positions in the indicator field.
If they are in seperate columns then you can simply import them into a new temp table then process the records to create new records looping through each column and setting the VariableName.
Most things are possible however often these "transform" processes need an indicator as to how to process disparate records. Because you are working with Excel it shouldnt be too hard for you to add a column that gives a script program some indication of "how" to process the record.
Assuming that each column, v1, v2, v3, etc is a differrent instance of the same data, you can use a series of imports. Import records the first time matching columns 1 and 3 to to the ID and value fields in your table, then use Replace field contents to load the variable field with the column header, v1. Then repeat the same process, but match columns 1 and 4, 1 and 5 and so forth until all the data is imported.
If this is something you will do more than once, you can script this process.