Assuming all you want to do is to insert 'nice', without the underlining, the simplest method is to use a Find/Replace with the expressions concerned:
Find = haveve a day
Replace = haveve a nice day
If you've copied/pasted 'haveve a day' into the Find box, you may as well paste it again into the Replace box as well and type in the 'nice'.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]