foreign data wrapper option manipulation during Create foreign table time?

hi, guys,

I am looking for a couple pointers here about fdw, and how to change the
option values during CREATE table time.

I am using postgres-xc-1.2.1 right now. For example, it contains file_fdw,
whose create-table-stmt looks like:
CREATE FOREIGN TABLE t1(....)
SERVER file_server
OPTIONS(format 'text',filename *'testing.txt'*);

I would like to replace the 'testing.txt' with absolute path like
'/user/testor1/testing.txt', and make sure the new value is saved in
pg_foreign_table; the file_fdw_validator is used to validate the options,
but is there a way to replace the optionValue here? And get the new value
stored in pg_foreign_table?

Thanks

BTW, in my real use case, I am trying to interpret a hdfs file and would
need to save some hostname/port information in the option value, which not
necessary specified by user.

Demai