If I hold on Power Nexus at 6 to 7, can I pay the energy cost to go up to 8 points and win the game?

Yes, you can.

According to FAQ #8834 and Rule 466.1.a.1, points gained from sources that are not "Conquer" or "Hold" (such as a card's activated ability) are not subject to the final point restrictions that apply when trying to win via a "Conquer" action.

Here is the breakdown of your turn:

1. Hold Trigger: When you move a unit to Power Nexus, you "Hold" the battlefield. This scores you 1 point, moving your total from 6 to 7.
2. Activate Ability: Because the Power Nexus ability allows you to pay a cost to "score 1 point," and this is a card effect rather than a "Conquer" or "Hold" action, it is not restricted by the "Final Point" rules (Rule 466.1.b).
3. Victory: By paying the cost to use the Power Nexus ability, you move from 7 to 8 points. Since you have reached the Victory Score of 8, you win the game immediately.

Important Note: Per FAQ #9177, while you can trigger the Power Nexus ability multiple times, you are limited by Rule 465, which states: "A player may only Score, from either method, once per Battlefield per turn." This means you can only successfully score the "1 point" from the Power Nexus ability once per turn, even if you move units in and out of the battlefield multiple times.