You could add Merge Cells to the Format menu and then type Alt, o, m.
Add Merge by going to Tools and click Customize. On the Commands tab, click
Edit. Find the Merge Cells command and drag it into the Format menu.
--OR--
You could create a macro and assign a keyboard shortcut.
If you are new to macros:
http://www.mvps.org/dmcritchie/excel/getstarted.htm
Sub MergeThem()
If TypeOf Selection Is Excel.Range Then
If Selection.Areas.Count = 1 Then
Selection.Merge
End If
End If
End Sub
One way to assign the keyboard shortcut: In Excel, go to Tools and point to
Macros and click Macros. Select the macro you want to change and click
Options. Assign the keyboard shortcut and click OK.