I have setup a somewhat similar case.
What I have done is store the identityName in a custom attribute when HR triggers the transfer (IdentityTrigger for a Mover) and have a rule creating a Mover certification.
You can do something similar, have the admin select the certifier and store this with the certifiee and date in a custom attribute.
In the rule decided if a certification is to be started and start it.
Look at the code in the thread Call certification event template in rule
The nicest thing about this solution is that the certification is created on the actual change date and not when HR triggers the transfer.
I hope this helps,
– Remold