Our scenario was replicate some Customers from ECC to CRM and assign them to a default custom role, as you already pointed the standard create them as sold-to-party, what I did is create a FM as copy of BUPA_INBOUND_MAIN_CENTRAL (for example) and inside I overwrite the Business role at the table c_bp_struct-central_data-role-roles. Once you got it you need to link your FM to the middleware events via SM30 - CRMC_BUT_CALL_FU. I don't know if there's a better approach but this one worked pretty well for our scenario.
Cheers!
Luis