How can I insert NULL into column with the type of timestamp?

H i ,

I want to tansfer a database from sqlserver2005 to postgresql8.2.5.I
use bcp to export a table named admin in sqlserver to a text file:

--table T_admin
id             name               key                  regDate
                         isLock          realName
1    rison          998877    2007-08-27 10:24:57    False               admin
2    lijun          778899    NULL                                    False          NULL
3    guanliyuan    111111    2007-11-05 10:30:08    False               myAdmin

   --admin.txt
id       name      key           regDate
  isLock        realname
1    ris    998877    2007-08-27 10:24:57.000    0    admin
2    lij    778899        0
3    guanliyuan    111111    2007-11-05 10:30:08.813    0           myAdmin

I created a table in postgresql,and I use "copy" to import datas

  create table T_Admin(
                adminID serial not null primary key,
                name varchar(30) null,
                key varchar(30) null,
                regDate timestamp null,
                isLock bool null,
                realName varchar(30) null
)

  copy admin from "/home/postgres/data/admin.txt"

Then error occured:
    error:invalid input syntax for type timestamp:""
    context:copy T_Admin ,line 2,column regDate:""
It seemed that the column REGDATE cannot accept a NULL.I tested it use:
   insert into T_Admin(name,key,regDate,isLock,realName)
   values('aaa','aaa','','1','aaa');
The same error occured.

  So it means that the column with type timestamp cannot accept a NULL
.Is there any way I can tansfer this table into postgre?How can i deal
with NULL in this case?
   Thank you!