None of the editors used in mojoPortal has any support for paste from Excel. Though it may work in some versions of the editors to some degree it is not a feature listed by any editor that I know of and pasting such content probably results in really bad markup in all cases.
The difference may be because we have upgraded to newer versions of the editors (CKeditor TinyMCE) and perhaps it worked better in older versions (not by design but by happenstance).
As long as you have not deleted the older versions of the editors, you could configure it to use an older version by overriding these settings in user.config
<add key="CKEditor:BasePath" value="~/ClientScript/ckeditor332/"/>
<add key="TinyMCE:BasePath" value="~/ClientScript/tiny_mce338/"/>
<add key="FCKeditor:BasePath" value="~/ClientScript/fckeditor266/"/>
you would need to look on disk under the /ClientScript folder to see what older versions you have there
However, no matter which version of the editor you use, I suspect that if you look at the markup after you paste it, it will be really bulked up and messy markup. I have not tried it myself, but pasting from Word also produces really bad markup and I would expect the same from Excel.
Hope it helps,
Joe