Sukri, a webkit report (if you are using report_webkit module) works with one ORM model.  You can still access the information from other models related to the master model.  So, if your master model or the webkit report is account.invoice, then you have access to it's partner_id (which is res.partner), it's lines (which is account.invoice.line), the line's account_id (which is account.account), etc.

Now, if you really need to combine unrelated models (though I'm not sure how you're going to combine it if it is unrelated), you can still develop a database VIEW and create a model with _auto set to False.  Something like used in analytical reports (addons/account/report/account_invoice_report.py is one example).