I kind of doubt that it really worked that way even in VB6. The code worked, but I would be surprised if they just inserted into an existing string. More likely, there was a bit more work behind the scenes that resulted in a new string that included the changes. So you can probably perform the same action, you just can't do it in a single line of the same nature. The line would look worse, something like:

SomeString = SomeString.Substring(0,1) & "zykx" & SomeString.Substring(1)

or somewhat different, depending on what that old Mid was actually accomplishing.