H1 B Transfer before joining employer

Can one transfer H1B BEFORE Oct 1 to a different employer?

(Basically the scenario: Approved H1B, not joined yet so of course no stamping.)

Also, can one file for multiple H1B transfers? meaning for more than one employer?

Yes, another employer can file a cap-exempt petition for you using the newly approved petition. The new transfered petition will also have a start date of Oct 1 or later.