Build up the relevant string with a for loop, like so:
Set objUser = GetObject _
sWorkstations = ""
For x=1 to 300
sX = right("00" & Str$(x), 3) 'Builds the string representation of x with leading zeroes
sWorkstations = sWorkstations & "COMPS" & sX & "," 'appends sWorkstations for use later on
sWorkstations = left(sWorkstations, len(sWorkstations) - 1) 'removes final comma
objUser.Put "userWorkstations", sWorkstations