I don't understand the logic of that issue
I just copy an effect and paste it with different name with same type. It should works IMO
I'm bit disappointed with this minimal documentation. And quite messed editor (too many same variable with no difference in their names)
Maybe i'm just bit too early in this modding and need more practice 
I am with you on the logic, it should work. But there are ALOT of things like that with GUTS. It is a great tool, but it can be rather limited on random things. Some of these issues are probably over-sites, some are probably on purpose so Runic can protect their IP.
It makes sense that you can't add Effects. Each Effect is a reference to a class (or function, if you will) in the code. To add a new Effect, you'd have to write a new class into the code, which we can't do in GUTS.
We can add entries to other .dat files because they reference arrays, which are lists of entries (like items or skills). The number of entries in an array is unimportant, so adding to them works.
And RnF's right that we probably can't modify existing Effects because that functionality wasn't included in GUTS.