Hi,
One way to create interaction is to check how report CRM_CALL_ACTIVITY_INTERFACE works (it uses BAPI BAPI_ACTIVITYCRM_CREATEMULTI).
other way is
For example, you can create it using CRM_ORDER_MAINTAIN and CRM_ORDER_SAVE function modules, or you can also create it using the BOL model provided in CRM.
Maybe the easiest way for simple data is the first way I've told you. But for more complex scenarios, this two last options are more complete 
).
Regards,
Sumeet