Sure, just define a list of characters that you want to be able to follow the character from your main string and insert a random one:
Code:
Dim s As String = "password"
Dim sb As New StringBuilder
Dim r As New Random
' list of valid characters to use after each character
Dim alphabet As String = "1234567890/;'[]\-+|"":?ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!@#$%^&*()"
For Each c As Char In s
sb.Append(c)
sb.Append(alphabet.Substring(r.Next(0, alphabet.Length), 1))
Next
MessageBox.Show(sb.ToString())