HPCC-10649 Allow legacy import and when to be configured separately

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
HPCC-10649 Update help text to cover new options

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5324 from ghalliday/issue10649

HPCC-10649 Allow legacy import and when to be configured separately

Reviewed-By: Jamie Noss <james.noss@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>