Membership - Use existing Useraccounts with no PasswordSalt

I have an existing user Database with passwords stored MD5-hashed.

The SQL -membershipprovider of ASP 2.0 stores Passwords with a salt -
value. I tried to set the salt of imported users to en empty string,
but then ValidateUser returns false.

Is it possible to use Passwords that are not salted?

regards
Mathias