I have no idea about it being possible through scripting. Perhaps.
But in the search bar you can't have both. UTF-8 is only UTF-8.
That's why i suggest you to search in byte instead of string. Because then you can do a group search mixing byte, word, dword, float, double and qword...but with string search you can only search for strings. So what you can try to do is search the string. Then gameguardian will show you the results. if you see the string twice copy the first 11 results and then you have your bytes. And then search for nearby values that do not change and copy them as a group search.
But i believe your main question answered. You can't do group search in UTF-8 or any other kind of string encoding using the gameguardian search bar.