This can be done using the ini format?
no, that's not possible as INI as a format with only sections and options is pretty limited (you can do awesome stuff with it as you can see with Rainmeter, but XML as a format makes much more sense for Rainlendar) and Kimmo stopped developing that skin format long ago.
there is one thing you should have in mind: XML as INI is just a method to structure data, it is not a scripting language. you can script Rainlendar using Lua scripts, but you really won't need that until you want to create really sophisticated stuff.
after all it depends on what you want to do. Rainlendar is capable of much more than the skins here at Wincustomize do (no offense, there's nothing wrong when people enjoy what they are doing, but almost all of them are paint by numbers). be it displaying more than one month, weather forecasts, rss readers, calendar and appointments in the same window, variables etc.
maybe it's worth learning something new, it might be even fun. 
