I haven't heard of needing a official letter to state what your approved chemicals are. Always used a chemical register, singled out what was food grade and what was not, and what was used for sanitation. Annual review of 11.2 & 11.6 should cover the upper management aspect of sign off. Adding the chemical to your list and maintaining records is part of the approval process. As long as you have a system for bringing in a new chemical that the whole site is following you should be all set.
11.2.5.2
Detergents and sanitizers shall be suitable for use in a food manufacturing environment, labeled according to regulatory requirements, and purchased in accordance with applicable legislation. The organization shall ensure:
i. The site maintains a list of chemicals approved for use;
ii. An inventory of all purchased and used chemicals is maintained;
iii. Detergents and sanitizers are stored as outlined in element 11.6.4;
iv. Safety Data Sheets (SDS) are provided for all detergents and sanitizers purchased; and
v. Only trained staff handle sanitizers and detergents.
11.6.4.1
Hazardous chemicals and toxic substances with the potential for food contamination shall be:
i. Clearly labeled, identifying and matching the contents of their containers;
ii. Included in a current register of all hazardous chemicals and toxic substances that are stored on-site; and
iii. Supplemented with current Safety Data Sheets (SDS) made available to all staff.