In the CSV file, is the standard description and the extended description (A) in the same field, with the first 30 characters always the standard description? Or, (B) the two are separated somehow, either by a char sequence within the field, or just in separate fields within the CSV? If (A), do you have the ability to edit the CSV, placing some type of separating character between the standard & extended descriptions?
If you can edit the description field in your CSV, to include a special character at the points where you want Carriage Returns (CRs), you can just refer to the field number in your formula without using a TEMP variable. For example, If your CSV file has the Extended Description in the second field, and you can enter piping ""|"" or some other unique char to represent your CRs, the formula SUB(IMP$[2],""|"",$0d$) should work when importing directly to the ItemCodeDesc field.