I believe you need to use a private key file to migrate the certificate that way. Here's a blog post I wrote about using certificates to sign stored procedures where I found that moving a certificate from one database to another user database required a private key file be created when backing up and restoring to the new database. Here's the basic syntax:

/*

Create the Certificate

*/

CREATE CERTIFICATE cert_access_other_db

ENCRYPTION BY PASSWORD = 'c3rtPa$$word'

WITH subject = 'Access Other DB'

GO

/*

Backup the certificate being sure to use a Private Key

*/

BACKUP CERTIFICATE cert_access_other_db TO FILE = 'C:\Certificates\cert_access_other_db.cer'

WITH PRIVATE KEY (FILE = 'C:\Certificates\cert_access_other_db.pvk' ,

ENCRYPTION BY PASSWORD = '3ncRyptKeyPa$$word',

DECRYPTION BY PASSWORD = 'c3rtPa$$word');

GO

/*

Create the certificate in the new database from the file

*/

CREATE CERTIFICATE cert_access_other_db FROM FILE = 'C:\Certificates\cert_access_other_db.cer'

WITH PRIVATE KEY (FILE = 'C:\Certificates\cert_access_other_db.pvk',

/*The password used to create the private key*/

DECRYPTION BY PASSWORD = '3ncRyptKeyPa$$word',

ENCRYPTION BY PASSWORD = 'D3cryptKeyPa$$word');

GO