You want to use a replace technique (in whatever language/environment you are using) on your substrings by capturing like this:
(\d{2})(\d{2})(\d{2})
*note the curly brackets are for improved efficiency.
And replace with:
$1.$2.$3
Here is a demo link.
Here is a SO page discussing the execution of replacements on nano.
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solved How to add dots between every two digits of a six digit substring?