Huh. Not working for me.
If I select text and cmd-c cmd-v right over the top of the selected text it replaces it without adding anything.
If I select the text and run the macro, it breaks the line.
I tried it in TextEdit.app and it works as expected.
Well, as I was preparing this message I was able to stumble myself into an Apple Script solution that works perfectly.
activate application "Microsoft Outlook"
tell application "System Events"
tell process "Microsoft Outlook"
-- insert GUI Scripting statements here:
tell menu button 4 of group 3 of scroll area 1 of tab group 1 of window 1 to perform action "AXShowMenu"
click radio button 69 of group 1 of group 1 of window 1