The String$ function can replicate only 1-char strings, so it seems that you need a loop to duplicate strings that contain 2 or more characters. However, this is a one-liner that does the trick:
Note, however, that - depending on the length of the string and the number of repetitions - this approach might be slower than an optimized code based on a loop.
Function ReplicateString(Source As String, Times As Long) As String
' build a string of spaces whose length is equal to the number of
' repetitions, then replace each space with the string to be repeated
ReplicateString = Replace$(Space$(Times), " ", Source)