here is a quick thought.
there is probably a much more graceful way to do this and this only works if these are the only 2 number types but try this.
put the formula one cell to the right of your data.
=IF(LEN(C8)=10,CONCATENATE(LEFT(C8,3),"-",CONCATENATE(MID(C8,4,3))),CONCATENATE(LEFT(C8,2),"-",CONCATENATE(MID(C8,3,3))))