Blank SQL insert statements

I am developing a system to take a generic data structure and store it in a
PostgreSQL database. The data structure contains several entries of the
following:

insert a record into table1
field1 value1
field2 value2

insert a record into table2
field1 value1
field2 value2

Data for different tables is stored in each 'data set'. When I get this to
the database I want to:

insert a blank record into tablex
populate fieldx of new record with valuex

I could implement this with some if statements based on the table name, then
INSERT INTO x (...) VALUES (...), but I would prefer to kept the structure
as generic as possible. Is it possible in PostgreSQL to construct a blank
record without specifying a list of fields and values in an INSERT statement
(so I can perform a collection for UPDATEs to populate the blank record)?

Ashley