The list of formats for property lists is not extensible.
The list is defined in the NSPropertyListFormat type/enumeration found in the
NSPropertyList.h from the Foundation framework (equivalently, CFPropertyListFormat found in
CFPropertyList.h from Core Foundation).
If you are looking for a plain text format that is easier to edit (easier than the XML format), then you may be interested in the older NeXTSTEP/OpenStep property list format. Unfortunately, it can not represent all the types that are supported by the new XML and binary formats (it lacks direct representations for booleans, numbers, and dates). Because of this forward incompatibility, the standard library and tools (NSPropertyListSerialization, defaults, plutil, PlistBuddy, etc.) will read this older format, but they will not write it (although the output of
default read looks suspiciously like the NeXTSTEP format).
This format is often found in the
.strings files that are used to localize applications (probably because this use case is limited to string keys and values in a dictionary (all supported types) and because it is easy to edit in virtually any text editor).