No, ServiceM8 doesn't support user defined job numbers.
There are numerous benefits to having ServiceM8 automatically allocate your jobs numbers for you. You will not have duplicate jobs because of staff mis-communication or staff in multiple locations booking jobs at the same time.
Depending on your requirements, you can also adjust the automatic job number to suit your business, by adjusting your current job # or adding suffix characters.
Submit a ticket to support if you need help with understanding how automatic job/invoice numbering will work with your business.