You do not have to merge a cell, you can simply increase the column width to accommodate the text you wish to enter. I never mentioned merging cells.
The only thing I can think of that sounds similar to what you're saying is to select your range, and goto Format Cells --> Alignment and in the 'Horizontal' drop down box select center across cells.
Another alternative you have is to place a text box from teh drawing toolbar on your document, allowing you to type in a similar vein to Microsoft Word.
What you have to remember is that XL is not designed for rich text formatting - its a number crunching app. If such issues are particularly imiportant, perhaps you should reconsider your choice of program for your task?