By default a MicroStation pen table does not allow you to string variables together but via a named expression you can easily accomplish this.
First open .dgnlib file where you are going to store your named expression
From the pull down menus select Utilities->Named Expressions
Select the New icon
Enter an “Internal Name” and an “External Name”. For this example we will use the same name “FilenameDateTime”
Under Keywords, check the box next to File and Print Set
Under Symbol Sets add “ActiveFile, PrintSet, Syetm.Path, System.String, PrintDefinition, System.DateTime”
In the expression field enter:
System.Path.GetDirectoryName (PrintDefinition.SourceFile) & "\" & System.Path.GetFileName (PrintDefinition.SourceFile) & "-" & System.String.Format("{0:G}", System.DateTime.Now())