Yes.
Just create a file in the mods folder, name it whateveryoulike.dm and edit it like a text file.
I'll just copy my test mod in here which I use to test the thugs I want to make. It's very simple and adds three spells, you can probably guess what they do.
Code:
#modname "Testmod"
#description "Mod for testing."
#version 1.0
#newspell "Test - Wish"
#copyspell "Wish"
#researchlevel 0
#pathlevel 0 1
#fatiguecost 10
#end
#newspell "Test - GOR"
#copyspell "Gift of Reason"
#researchlevel 0
#pathlevel 0 1
#fatiguecost 10
#end
#newspell "Test - Teleport"
#copyspell "Teleport"
#researchlevel 0
#pathlevel 0 1
#fatiguecost 10
#end
The
bold part (mod info in the manual) must be always at the start of the file, so the mod is recognized.
After that you can get on with the actual modding. To change the attack of say a Golem to 18 and defense to 3 you'd do the following mod.
Code:
#modname "Golem mod"
#description "Mod for golems."
#version 1.0
#selectmonster "Golem"
#att 18
#def 3
#end
edit: Sometimes you must watch out as the units have the same names.
For example the indy druid and the marverni druid. You must use the unit id instead of the name then.