If you're working as a locum for a practice on a self-employed basis, have been added to the contract as a performer and you're not employed through an agency, you would be eligible to pay into the NHS Pension Scheme.
Your Net Pensionable Earnings (NPE) will be declared at the end of the financial year on the Annual Reconciliation Report (ARR).
If you're working as a locum through a third-party such as working for an agency, you would not be eligible to join the NHS Pension Scheme as you're not employed by the NHS.