I would suggest using the "edit" option on your original post if you need to add more information than the ~250 characters will allow.
To differentiate that information from your original post, add it at the bottom, and you could start it off with a:
or, if it is directed at a specific persons' answer, you could do:
To assure they see you reply (through their profile area), I would suggest adding a comment to their post directing them to check the original question for your updated information.
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