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:

http://doc.i-doit.org/wiki/Manual

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.