Create an action: Press Alt-5, Press the "New" button in that dialog, enter a meaningful name for the action e.g. "Insert hyphen after number".
(this was the general how-to for actions, now comes the more specific part)
Create an action of the type "Replace with regular expression.
As field select _Filename
as regular expression enter:
as replace pattern enter:
(please note that there is a blank after the hyphen!)
Store the action by pressing a sequence of OK buttons.
Select a file for test-conversion, select the action from the toolbar-dropdown-action-list.
If the result is satisfactory let MP3tag do the work for you.