reftex-format-cite-function

reftex-format-cite-function is a variable defined in `reftex-vars.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
Function which produces the string to insert as a citation.
Normally should be nil, because the format to insert a reference can
already be specified in `reftex-cite-format'.
The function will be called with two arguments, the CITATION KEY and the
DEFAULT FORMAT, which is taken from `reftex-cite-format'. The function
should return the string to insert into the buffer.

You can customize this variable.