I am not aware of any job positions within the Libertonian law enforcement that would allow you to go outside Liberty. After all, it's called -Libertonian- for a reason. The only one group coming to mind is the LSF, but I would highly advise not joining the LSF. They are not exactly known for their good reputation, and joining them would certainly have an effect on your own reputation.
Naturally, joining the law enforcement would result in you not being able to fulfil contracts Sirius-wide. You can, of course, still escort traders coming in and out of Liberty but only within Liberty house space.