The student wrote:

Is this possible?

Depends on how hard you want to work. The ATP "Anova: Single Factor"
tool handles unequal numbers just fine. With multiple factors, you will
either need a 3rd party add-in or will have to manually implement the
formulas.

If you are trying to implement calculations for a very specific design
that will be used repeatedly by a number of different users who all have
and are comfortable using Excel, then it might be worth manually
implementing.

Otherwise, a far more flexible approach would be to obtain and learn a
full-blown statistics package. R is an open source implementation of
the S language that may be freely downloaded from
http://www.r-project.org/

Jerry