The Copy permissions article property is not exposed via the Article Properties dialog, you must add the article to the Merge publication using sp_addmergearticle and include @schema_option 0x40000000 which will copy permissions.
I've tested this in SQL Server 2008 and it works perfectly. However, I've tested this on SQL Server 2012 and there is a bug which prevents the permissions from being copied to subscribers. In that case, you would need to workaround the issue using a post snapshot script or post the GRANT statements to subscribers using sp_addscriptexec.