Can I add yeast energizer and sugar to my secondary fermentation?

You can always add "stuff" (pumpkins/sugar/hops/etc) to a brew at almost any stage of the fermentation. It is more efficient to do it in one go but for the home brewer it is a matter of choice. The only negative possibilities are secondary infection from nonsterile ingredients or opening the FV. The easy way to get round that is to pasteurise any later additions. Pumpkin can be pulped and microwaved, sugar can be dissolved in a minimum quantity of boiled water. Even hops can be microwaved - although I stress that is not usually needed. I don't think anything else will help - IMHO "energiser" is a but of a misnomer. The yeast is either vigorous or it isn't. The brew either supports fermentation or it doesn't.

However be mindful of the fact that just adding stuff (esp fermentables) can have a limit. If the OG was "too low" then more dextrose or sugars can be added later. But the outcome will depend on the yeast attenuation rate. The yeast will only ferment to a certain point. Adding sugar beyond that point will result in a sweeter brew - or one that can be further fermented during conditioning by invasive species (like Brettanomyces or Pedioccocus).

Any additional fermentables will add to the ABV; however, be careful with the yeast you're using because it's tolerances may be lower than you need and you may end up with a sweeter beer that you're expecting. There's no reason not to add these fermentables to the primary if you want to extend the fermentation process unless you're trying to get it off of the cake (which you really don't need to worry about with newer strains of yeast).

I don't even do a standard "secondary" anymore. I do an extended primary if I need extra fermentation and use the "secondary" time for conditioning/fining/flavorants.

When adding fermentables and nutrients this is basically a feeding and will raise the ABV. Usually done while on the primary yeast.

Personally because the pumkin may have some fermentables I would do this late in primary and use your secondary just for fining. If at all.