You're not actually using regular expressions. Your search pattern is just a plain string, so you'd be better off using strrep.
The reason it doesn't work with Add as a search pattern is that the ( character has a special meaning in regexes so to match a bracket you need to escape it with a backslash, either manually or using regexptranslate:
regexprep(Text, regexptranslate('escape', Add), Search)
But as I said
strrep(Text, Add, Search)
would work just as well and will probably be faster.