Actually, the required functionality is implemented (syntax is An) and documented.
However, since it was only recently implemented (May 28, 2019), you probably don’t have the current patch of the KOM process.
Description of D2D-4994 functionality:
In the Modbus Client protocol, addressing of “big-endian” strings was supported, i.e. strings having 1 byte per character and sent in a “big-endian” order, i. characters of “123456” are sent in the order of “213465”. Addressing is supported by An syntax, e.g. A10 is a 10-register string, i.e. 20 characters.
Until now, the addressing of “little-endian” strings (syntax an) and the addressing of strings that have 2 bytes per character and are in “bit-endian” order have been supported.