Yes it is possible, keys can be regulated to a certain weight, 50 grams etc. this can be changed by altering the lead weights in the sides of the keys.
However, do not assume that it is that simple. It is possible that your piano may have a sluggish action due to swelled center pin or key bushings. This is what usually tires out the hand the most, and is very frustrating to play.
The cheapest thing to fix is the key bushings, and the weighting, or centerpin bushing replacement would be quite pricy if done right, ( I would charge between 500-1000 dollars CDN for that work.) (center pins)
It may be that you are ready for a new piano.
In that case, I have a piano that has a glorious sound, and a light action. (almost to a fault.) Ihave not had a chance to get around to weighting the keys a little, but when I do, it will likely be the best upright I have ever played. In it's stock form it is quite light, but you may really appreciate that...
I have a seiler Primus. Go check it out.