Are you talking about displaying information to the user to tell them how to make their payment offline? Or are you talking about submitting payment details that can be processed offline?
The pay later thanks message is intended to provide any instruction needed after someone registers using pay later. You can of course put more info in the email also. To collect offline payment details for you to process manually, you'll have to create new fields for that data... but there is not a way to make new fields appear on the payment page itself just for pay later.
I really could use an option to give additional information/instructions about the payment types. For example, if the the user pays with iDeal but closes the window to quick after the payment, the information about the payment is not collected by DT Register en theirfore the registration info is lost.