I built an Index using Type&Tables>Index however there was a lot of additional work that had to be done manually after it was built, so I do not want to use the Replace command. Is there a grep expression I could use in Find Change to find the "tab + page number" and replace with "tab + page number adding 2" (i.e., so an item originally indexed to be on page 13 would then be indexed on page 15).
The answer to your original question is "no". Regular expression (the RE in GREP) is purely syntactical and doesn't work on a semantic level. To it, "12" is a series of two characters which happen to be digits, it has no way of interpreting it as the number "twelve", let alone processing it any further with arithmetic operation like addition.