Kirsten:
My first thought is that FrameMaker supports the identification of separate autonumbering counters ala "N:" where "N" is a case-sensitive (i.e., "N" does not equal "n") alphabetic character that specifies a unique autonumbering counter.
With that in mind, would it work to (a) create a separate paragraph style for the numbered items that require the long dash, then (b) in that style reference the autonumbering counter in conjunction with "<n=>________________"?
In other words, repeat the value of a given autonumber in a separate paragraph style and suffix that instance with your dash.
Otherwise, I personally know of no other way to inform FrameMaker that a given autonumber requires repeating AND should have a dash suffixed to that repeated number...(?)
Cheers & hope this helps,
Riley