Do not EVER use a power supply with adapters in order to run a single card that needs more than it has, or dual cards.
As mentioned, if the PSU was capable enough to run a specific card or cards, it would already have the connectors. Even some power supplies that have the correct connectors may not be good enough for a given setup, but if it doesn't have them at all, then it's a really bad idea.
If your PSU has only two 6 pin connectors, it's ABSOLUTELY not good enough, nor high enough capacity, to run GTX 980 SLI.
What is the model number of your power supply?