V_T5G_PRFID is the Table View of a system table with a delivery class of 'S' (System table, maint. only by SAP, change = modification) meaning it is not for customers to modify.
However, for V_T5G_PRFID, the acceptable/allowed Proof of Identity document types - as per UKVI (UK Visas and Immigration, used to be called UKBA - UK Border Agency) rules - are specifically the entries that are delivered in SAP standard as per the relevant legislation. If new entries are defined (by UK legislation) which are valid for the UK, these will be delivered in SAP standard. The reason for the table being an 'S' table is that the documents for Proof of Identity that are allowed are based on the rules dictated by the UKVI, so customer-specific entries are not allowed.
In addition, T5G_PRFID is a UK support-owned table and is only used in the UK screen 2008. Only employees employed in the UK should have data entered in IT0048 (IT3292) using screen 2008. If e.g. a customer has a UK employee that has Singaporean nationality, then their eligibility to be employed in the UK is governed by UK law and so the customer must use screen 2008 - and proofs of identity delivered by SAP - to record their residency status.
If an employee is employed somewhere else in the world other than the UK, then they will have a separate personnel number (for that country version) and their residency status information (if applicable to the country version) must be entered using the correct screen (for that country version), which won’t be the UK-specific 2008.