Unfortunately no. The Job ID and Subjob IDs need to be numeric values and will automatically assign when a new job and subjob is added.
There is an alternate code field which allows you to manually assign a code to the job or subjob which can be alphanumeric.
You will need to expose some fields (if they aren’t already), by using the Form Settings button.
Expose the Alt Code field on the My Jobs, Job and Subjob screens.