Is there a way to simply add a common tag, using multi-edit mode, to selecting existing tags, without overwriting their current content?

For example:

Item A: tag1 tag2 tag3
Item B: tag4 tag5 tag6
Item C: tag7 tag8 tag9

Is there a way to add tag10 to this so that you get:

Item A: tag1 tag2 tag3 tag10
Item B: tag4 tag5 tag6 tag10
Item C: tag7 tag8 tag9 tag10

And not:

Item A: tag10
Item B: tag10
Item C: tag10

?

If not, and if one of the RTM people are here, could you please consider in some way altering the way multi-edit mode works so that this could be done?