How do I convert double to string?

I need to auto populate an Id field with 999999991, then 999999992, etc. when an Id is not entered by the user. The Id is text, since the user wants to use a leading 0 in the Id, such as 043-432-098. So, I am converting the text to a double, so as to be able to add 1 (when I used int, I got an overflow error). Then, I need to convert this new number, 999999992, for example, back to a text string. My code looks, as follows:
Dim MaxForcedANumber As String
Dim ANumberDbl As Double

'If ANumber was not entered, force 99999999x
If IsNull(Me.ANumber) = True Then
  'Search for largest A Number in database (forced A Numbers are largest)
  MaxForcedANumber = DMax("ANumber", "tbl_AllClients")
  ANumberDbl = CDbl(MaxForcedANumber) + 1
  Me.ANumber = Mid(ANumberDbl, 1, 3) & "-" & Mid(ANumberDbl, 4, 3) & "-" & Mid(ANumberDbl, 7, 3)
 End If
I get a type mismatch error. What is incorrect?