Thanks for your posting! We recommend the CSV import to migrate test cases from other systems:
The CSV import was specifically designed for this and most tools support a CSV and/or Excel support you can use for this. For example, for TestTrack I found the following KB article which looks like a good starting point:
Once you have an Excel file, you can simply save this to a CSV file in Excel and then import this directly using TestRail’s CSV import wizard:
I hope this helps but please let me know in case you have any further questions.