You can create a calculation field that does the following calculation:
Assuming that your current job number field is called "JobNr"
Right ( JobNr ; 5 )
That should give you the 5 right digits from the Job nr. field.
I would also recommend that you NOT use the calculation field nor the original alpha numeric value as your primary key to link this main table to other related records. Use an internally generated value such as an auto-entered serial number or Get ( UUID ) instead. This frees you from future changes to the "official job number" from affecting the function of your database. You can keep that value as a text field in your main table so that you can use it to sort and search out records but changing the value won't affect what records in other tables link to it.