Well I didn't get to use this great tool until today, but it looks like custom CAS makers will need it now...
I found out that when editing a CAS with seasons installed, the created sims freeze whatever the current season is in game or when editing the lot - possibly to death since my last guinea pig fell frozen to not get up afterwards like the others did before, and the game crashed on exit. Of course adding a roof doesn't change a thing, and SimPE didn't show anything readable that could be related.
So I finally edited my custom CAS with OFB, and it now works fine in Seasons.

The fact that I need to uninstall the base game starter in order to play my full game isn't a big issue - most likely related to the fact that I play the english version of the game on a french computer
Oh and I love the mini-game, it's sooo cute!! lol