The best way is to define it in the script, but you should use a loop so you only have your dimension defined in one place:
For each vFile in FileList('C:\Path\Transactions*.csv')
FileName() as FileName,
<SomeFormula> as NewDimension
From "$(vFile)" (...) ;
But it is also possible to do it in a chart or in the Qlik Sense asset panel. Just start writing a formula with a leading equals sign.
See also Loops in the Script
Another way is to create the dimensions after the concatenated table is created, through
<Load all CSV files in order to get MyConcatenatedTable>, followed by
LEFT JOIN (MyConcatenatedTable)
MyFormula as NewDimension
Just be careful on performance, depending on the amount of records.