Things doesn't work like this, replication between CRM and ERP should be via BDoc as long as is possible, doesn't make sense to map an IDoc to a BDoc. The only exception is in integration scenarios, where you are not replicating the information: you do something in one system and this action should do another thing in the other system, this can be done via IDoc, RFC, Webservice, etc.
If you want to know how to replicate BPs or custom tables using BDocs you will find a lot of posts, articles, blogs in scn, is a very common topic. In other hand you want to convert a BDoc to an IDoc and viceversa you can use the XIF adapters, there's a lot of information in scn as well, but I don't see the point to use XIF when you are replicating info from CRM-ERP o viceversa.
Cheers!
Luis