Since import data could only import to stand-alone fields instead of subtables, you'll need to create a new sheet which displays subtables as stand-alone fields. Firstly, you would have to create a linked form from your subtable. To do this, select New sheet from subtable under the Sheet Linking Tools in the Design Mode.
Apart from containing all the fields in the subtable, the newly created form sheet will contain an additional field named after the sheet containing your subtable. This field will show the main entry that this subtable entry belongs to. You should take care how this data is mapped while importing your data.
Importing to the generated form will populate the subtables in your original sheet according to this linked field.