Pension Options
Most scheme rules dictate that an 'ordinary' employee contribution will be matched by the Employer contribution i.e. you won't get one without the other. Anything above that will be treated as an AVC, which can be used to buy an ARF. Most employer schemes are designed for groups and as such won't alter the rules for an individual, e.g. to allow all Employee as an AVC.
As for paying into the Personal Pension Plan, if all your income is PAYE and you have no other earnings, then you cannot contribute to the Personal Pension Plan. However as far as I understand it, you should be able to establish a PRSA if you want to maximise your AVCs - but bear in the mind the cost of this vs maximising your AVCs in the Employer Plan.