Thus, if that skill panel happens to be full already, the level is unsolvable as of now.
It may be worth waiting for the next NL update, which
should increase the skill limit to 10 types per level. Of course, it's up to Nepster whether or not this does get integrated for the very next version, but it's a very highly-demanded feature, and the code's all ready to go (aside from perhaps needing a tiny bit more testing).
Lastly, there are three levels in which zombies need to walk into radiation and/or slowfreeze, and those cannot be replicated even with additional skill slots and pickup skills. I still have no idea how to go about adapting those.
In my now-discontinued pack Doomsday Lemmings 2, there was a level "Guardian Angel" that had a crowd of normal lemmings, a ghost, a zombie, and a trap. You had to save every normal lemming. The solution was to use the ghost, to direct the zombie into the trap, rendering it safe while lemmings walk past. (It helps that the Horror set trap has a VERY long animation, so there's a relatively
huge window of time to slip the crowd past.) This part of the solution is fairly obvious so I don't consider it spoilery; the real puzzle is how to actually
achieve that. (I actually consider this level to be the only ghost level I made that was actually really good.)
Perhaps a similar setup can work for your levels - using some kind of object that the zombie keeps busy for the crowd - could help.