Replace zero length mail merge field with user input - vba

Question

I have a MS Word template where I have certain mail-merge fields and one of those fields in the source doucment is sometimes empty. I am using below IF condition and taking user's input for the date field but I cannot update the source mail merge document with this information.
If Len(ActiveDocument.MailMerge.DataSource.DataFields("startDate").Value) < 1 Then
InputStartDate = Format(InputBox("Enter Commenced date ..."), "mm/dd/yyyy")
ActiveDocument.MailMerge.DataSource.DataFields("startDate").Value = InputStartDate
End If
I get an error here that says mail merge source doucment field connot be edited or is locked, something like this.
Is there any way through which I could either update the source field and through application refresh method, I populate that mail merge field with this user input date?
Alternatively I could otherwise add a few lines of code to perform following steps:
1- Save this document (using Saveas2 method) as ".docx" by converting it from ".docm"
2- Break the mail merge connection through VBA code
3- Select all mail merge fields and using Sendkeys "CTRL+SHIFT+F9" covert all of them to normal strings.
4- Make that particular mail-merge field a Bookmark field and then finally update that bookmark field with the user input.
This sounds like a longer route of getting a simple empty mail merge field issue fixed but this is so far all that I could think of. If you have any better suggestion, please let me know.

Answer 1

Instead of vba, do this with fields. Then it becomes part of the Mail Merge.
{ IF { MergeField startDate } > 0 "{MergeField starttDate }" "{ FillIn "What is the Start Date?" }" }
This will trigger the Fill-In field unless there is content in the startDate field for the record.
Here is documentation on the Fill-In Field.
Here is documentation on the IF Field.
Here is my tutorial on the Ask and Fill-In Fields.
Here is my article on Dealing With Fields in Microsoft Word.
Here is Word MVP Paul Edstein's Tutorial on Mailmerge Tips and Tricks.