We could probably use a bit more information about the situation you’re facing. Implicitly, it sounds like you have an existing document (native odt?) with consistent formatting to which you now wish to apply level/heading styles – is that right?
Are you setting levels for whole paragraphs? or simply headings? I can’t think of another way than using paragraph styles for this, but using styles certainly gives you your “level 1 as bold, level 2 as italics, etc.” Of course, if you’re creating a document, you can use these styles on the fly.
Applying these styles to your current document is more tricky. The best way I can think of is to use the AltSearch extension. How you might use this powerful but slightly complex extension to automate your conversions is another matter, and it would help if you could make a sample document available (Dropbox? Google drive?), or show us (screen capture?) what it is you’re working with.
This might not tell you anything you don’t already know, but adding more information might elicit higher quality help.