Does sp_addtabletocontents Use Precomputed Partitions
Intriguing question... as long as you have an actual scenario to apply it to it wouldn't be too difficult to figure out.  Perform your BCP process, run sp_addtabletocontents and then before the subscriber syncs verify the existence of the records in the MSmerge_current_partition_mappings and MSmerge_past_partition_mappings tables.  

According to this technet article that is where the partitioning data for each row that has been changed is stored post change/pre sync.  Look into the article for deeper information and the manner in which the table structures are organized within the process.  The link provided is for SQL 2008 but if the table structure is the same its a safe bet that the pre-compute logic is close enough to answer your question.

I am not an expert on precomputed partitions but it seems to me that it would have to take advantage of the precompute process (if enabled) otherwise it wouldn't sync these records at all.