You can set text in content controls with a userform just as easily as you can bookmarks or anything else.
However if you want to keep patching up an old unruly document that is your business. Say you have three ask fields bookmarkd name, age and IQ.
Create a userform with three text fields txtName, txtAge and txtIQ and a command button. Use this as the cb click procedure:
Code:
Private Sub CommandButton1_Click()
Dim oRng As Range
Dim oBM As Bookmark
With ActiveDocument
Set oBM = .Bookmarks("Name")
Set oRng = oBM.Range
oBM.Range.Text = txtName
.Bookmarks.Add "Name", oRng
Set oBM = .Bookmarks("Age")
Set oRng = oBM.Range
oBM.Range.Text = txtAge
.Bookmarks.Add "Age", oRng
Set oBM = .Bookmarks("IQ")
Set oRng = oBM.Range
oBM.Range.Text = txtIQ
.Bookmarks.Add "IQ", oRng
End With
Hide
End Sub