https://docs.microsoft.com/en-us/sql/t-sql/statements/alter-table-transact-sql

Something like this:

ALTER TABLE Customers ADD 
    PromotionCode nvarchar(100)  
    ENCRYPTED WITH (COLUMN_ENCRYPTION_KEY = MyCEK, 
    ENCRYPTION_TYPE = RANDOMIZED, 
    ALGORITHM = 'AEAD_AES_256_CBC_HMAC_SHA_256');