If you want to use you Regex search and replace for this, something along these lines might do the job.
Given that you don't have any other methods named AddToX(
you could use the following regex:
\.AddTo([^(]+)\(
This would match .AddToX(
then you replace with .\1.Add(
where \1
is a back-reference, so it might be $1
depending on your flavor of regex.
That would transform .AddToX(
into .X.Add(
.
How you would process your code with the regex I can't answer so that would only be half the puzzle, many IDE's have project wide search and replace I guess.
Regardless, make backups before you do anything of this nature.