[rdonj]

Well, you can make a new item, but there's not much you can do with it. Item modding is mostly for changing the costs and requirements to make said items.

Edit: Let me work up a chassis for you real quick.

#newmonster anumberfrom 2200 to 2999
#name "Clam Master"
#copyspr monsternumber
#descr "A unit that produces clams."
#ap 10
#mapmove 3
#hp 10
#prot 0
#size 2
#str 10
#enc 3
#att 10
#def 12
#prec 10
#mr 14
#mor 12
#gcost 0
#rcost 1
#weapon "Useless Kick"
#armor "Robes"
#gemprod 4 5
#poorleader
#stealthy canhaveavalueforincreasedstealth
#amphibian or pooramphibian
#flying
#end

That's a relatively generic weak sort of chassis for you clam producing summon, a bit extra mr to make it slightly les vulnerable to kill spells and a little extra morale for no good reason. As far as the spell portion of your clam factory summon I can't really help you there, I haven't touched spell modding yet.

As for the food supply summon, you can do basically the same thing. Just replace the "#gemprod 4 5" line with "#supplybonus whatevernumberyoulike".

You can also remove all their item slots if you like with #itemslots 1, or give them no leadership with #noleader.