With the inception of smart grid technologies, electric utilities have been attempting multifarious mechanisms to transport information from data generating entities to their information control centers. One such approach for carrying application data is to lease communication network capacity from Internet service providers (ISPs). Such dedicated capacity reservations on communication networks can be a costly proposition for electric utilities. This work proposes an alternative paradigm that takes advantage of the elastic nature of smart grid data traffic flows to piggyback (with authorization) on ISP networks without impacting the QoS of the primary flows on that network. Such an opportunistic scheme is expected to reduce operational costs for utilities to carry smart grid data traffic while providing guarantees to ISPs that their primary flows will be unaffected. The proposed opportunistic network sharing scheme builds upon a software-defined networking (SDN) approach at an intermediate router to provide QoS guarantees for regular ISP traffic. To mitigate the inevitable throttling of smart grid traffic during times of high network utilization in the shared network, a data granularity management algorithm is proposed that gracefully reduces the data granularity of smart grid (SG) data traffic in preemptive fashion in an attempt to meet application QoS needs.