Final answer:
Christians added a transept to the Roman basilicas, giving the structures a cruciform shape to symbolize the crucifixion of Jesus and accommodate their religious practices.
Step-by-step explanation:
The primary addition Christians made to the basilicas they adopted from the Romans was the introduction of the transept. This architectural feature, running perpendicular to the nave and located between the nave and the apse, gave the building a cruciform shape, symbolically reflecting the crucifixion of Jesus Christ. This transformation from a standard Roman basilica to a Christian basilica served not only a symbolic purpose but also catered to the Christians' need for larger interior spaces to accommodate congregations and conduct religious ceremonies that were distinct from pagan practices.