Almost anything can be done "in Excel" if you are willing to program in VBA
;-).
There might be good free or for-payment solutions available. I don't know.
For my own purposes, I wrote my own macros that encode/decode using a fairly
simple algorithm based on a user-input passphrase. Although the algorithm is
simple, there are enough variables involved so that it is difficult for most
people to crack, I think. However, it has never been tested for
vulnerability.
If you are interested, send me email at joeu2004 "at" hotmail.com.