While I'm not convinced this is a good idea (are you sure these don't outline separate residences?), I do have a method that should work:
1. Select a all of the nodes along one side of the building except the last, and copy-paste them offset so that you have nodes roughly where you want them (or create them another way).
2. Use the [conflate plugin][1] with the "replace geometry" box un-ticked to copy all tags from the surrounding ways to the destination nodes (full instructions in wiki)
3. Select the destination nodes and delete the irrelevant tags
4. Select the outline ways and *Join Overlapping Areas* (SHIFT+J).
(SHIFT+J), selecting "**none**" for the conflict resolution on the address tags.
5. Use the *Simplify Way* (SHIFT+Y) command to delete midpoint nodes (you may need to tighten the sensitivity)
6. Remove the irrelevant tags from the outline way.
way. (For those not done as a part of step 4).
7. Modify the building tags on the outline way to reflect the fact that they are [building=terrace][2] or [apartments][3] as appropriate.
8. Unless specifically against local practice it may be useful to place the new address nodes joined to the new way and tag them as [entrances][4] if you know where the doors are.
[1]: https://wiki.openstreetmap.org/wiki/JOSM/Plugins/Conflation
[2]: https://wiki.openstreetmap.org/wiki/Tag:building%3Dterrace
[3]: https://wiki.openstreetmap.org/wiki/Tag:building%3Dapartments
[4]: https://wiki.openstreetmap.org/wiki/Key:entrance
