I've never tried this but have you tried:
Otherwise if you have a table of all the folders you can create a loop.
Something like below. I haven't tested it but it should work.
LOAD * INLINE [
For Each vFolder in FieldValueList('FolderName')
let vFileName = chr(39) & 'Source\' & vFolder & '\*.csv'& chr(39);
(txt, codepage is 1252, no labels, delimiter is ',', msq);