Solution #2 for todays problem
Have not tweaked spreadsheet formulas for a long time so this was a fun project.
With your list of imported numbers in the "A" column starting at A1 the following formula can be entered into B1 and copied down:
=CONCATENATE(LEFT($A1,3),"-",RIGHT($A1,4))
To adjust placement of the "-" the ,3 and ,4 can be changed accordingly.
For example if you want the dash after the first two digits the formula changes to"
=CONCATENATE(LEFT($A1,2),"-",RIGHT($A1,5))