They probably did, BUT most Slavs that migrated to the Balkans came from modern day Poland. Polish people today are one of the most (if not THE most) Slavic-looking people. Poles also have the highest rate of R1a, which is in most cases correlated with Slavic migrations. Of course not ALL R1a clades are slavic, but the vast majority found in Poland and the Balkans are of "Slavic" origins.Common Slavs culture comes from 5th century at the time of their expansion. Are you sure that at this time they didn't already include groups of two, or more, different physical characteristics?
If the Balkan Slavs were overwhelmingly Slavic, more of their haplogroups (other than R1a) would be present in their ancestral tribal lands in Southern Poland.