Hi Rohan,
there are quite a few blogs and threads regarding the topic of custom data replication using the CRM middleware (e.g. Steps to follow for the Replication of custom tables related to BP from CRM to ECC using Middleware). While they not refer to exactly your use case they still contain valid information.
In principle there are two possible approaches. Replicate the data together with the BP (this is I guess the approach suited best for your requirement) or replicate the data as a separate replication object. If you choose the first approach you need to do the following steps:
1.) Extend the BDoc data structures (e.g. BUS_EI_EXTERN)
2.) Implement a function module in ECC that extracts the data and stores it in the extended structure. Use function module sin CRMC_BUT_CALL_FU as an example
3.) register this function module in table CRMC_BUT_CALL_FU.
4.) develop a function module in CRM that extracts the data and create the marketing attribute for the BP.
5.) register the function module in CRMC_BUT_CALL_FU
6.) Depending on how you extended BUS_EI_EXTERN you might also need todevelop a function module that maps the BAPIMTCS structure to BUS_EI_EXTERN in CRM
Hope this helps,
Christian