What regex variant are you using? Regex libraries are not all the same and they provide differing functions and pattern syntax. The following is based on the assumption that your regex will have the necessary functionality.
There are 2 ways you can do this. The easiest (perhaps) is to look to see if there is a ".replace" function as well as a ".match" function. If so, then something like:
NewString = TextStr.replace(/\bAS TRUSTEE FOR\b/, "($0)")
would first locate the text you are after and then replace the whole lot with the same text surrounded by parentheses.
The second may is to use the information returned by the ".match" function to provide the offset and length of the match and then use the string functions of your language to insert the parentheses yourself.
If this is the only thing that you are doing using a regex, then you may find it simpler to use the string functions in your programming language to avoid the overhead of the regex engine code.