Hi there,
If I understand correctly you want to encrypted SiteMinder passwords, as well as the password data (last login, password history and password change time) to be migrated to IDM 12.6 SP5.
This is not a straight forward task and it very much depends on how you have configured your SiteMinder password decryption.
Accroding to Password Formats - Apache HTTP Server Version 2.2 These could be "CLEAR, SHA1, MD5".
If the password was salted this adds complexity to the task.
If I understand correctly, SiteMinder password storeage is in hex and IDM is in Base64
So like the example in the article Convert Base 64 encoded data to ASCII Text the text of "Mary had" is converted to "4D 61 72 79 20 68 61 64" of hex and "TWFyeSBoYWQ=" of Base64.
In order to achieve this you would have to retrieve the hex format passwords, and then convert to Base64 and store them again.
I think our CA Services have some experience in performing this task and it would be worth while to contact them in order to achieve this.