Is it possible to add WSS as front end and MOSS as server?
If you mean to have a MOSS box as ONE server - and then a WSS box for the Web Front End - that would be seen as "avoiding licensing" by Microsoft.
You could certainly have a farm of WSS boxes - if that's what you mean. ie. load-balanced and multiple WFE's - just using the free WSS components.
Licensing is an annoying + tricky game - contact Microsoft to check if your're covered, etc...