Unfortunately it is not possible to connect i-doit and OTRS. But we planned that a while ago so that there will be one one day (there is no date I can tell you). It is possible to accelerate it, by delegating us with that job and defined requirements.
Alternatively there is actually an interface to Request Tracker from Best Practical.
i dont think anybody will answer your question because you missed few words in it. Is your concern if there is difference between i-doit and iTop? There are differences between these products since they come from different companies. Have a look on features for each product to see details.
There are two possible ways to do that. You can use the internal workflow system to generate 'tasks' (doing something once) or 'checklists' (soing something on a regular basis). I think there should be a basic documentation available in English. You should have a look here:
Otherwise you can connect the Request Tracker (RT) from Bestpractical and i-doit in a way, that they can look in and reference each others databases. That means: While creating a ticket in RT you can reference items in i-doit. And, vice versa: in i-doit you will see the items and their corresponding tickets.
So, you have to decide which way fits your needs better.