I've branched this to a new thread of it's own.
Be prepared for an onslught of people telling you that this is bad style, though. We usually go the other way and remove double spaces after punctuation.
That said, this is not a simple as you might think since there are may things that can end a sentence, and some of them might occur in the middle of a sentence as well. ID thinks the period ends a sentence, for example, and you might be tempted to search for a period followed by a space and then a capital letter, but that could also include names that use a middle initial, and it would leave out sentences that end in a question mark or a quotation mark.
What's the reasoning for wanting to do this?
Please, don't! Microtypography does not allow it!
Even if it is bad typography you could extend space between an dot and the next character by using a GREP-characterstyle. This would also be useable for abrevation where a smaller space between the dot and the next character is correct (at least if I typeset German texts: u.s.w.; z.B. e.g. and others)