I bought bar stool chairs but they are too short for the bar area. Is there a way to make the chair stools longer in length?
Um, not really. Unless you got the stools from a mix & match furniture store like Ikea where you can buy the legs separately from the seat.
There are approximately 3 standard chair/table height combinations:
Table height (e.g. a standard dinning table; seat height ~18 inches, surface height ~30 inches)
Counter height (e.g. a kitchen counter; seat height ~24 inches, surface height ~36 inches)
Bar height (e.g. a bar or "high-top" tables; seat height ~30 inches, surface height ~42 inches)
Sounds like you have "counter height" stools with a "bar height" surface.
The only solution I can think of would be to build a box or frame of some sort, braced for stability and with the chair then firmly attached to it. Good luck finding a way to make that lightweight and attractive; I don't think it's I possible but I doubt it's much more practical than rebuilding the chair from scratch or buying new.
I suppose one way to "justify" it would be to add wheels as part of that frame and get most of the added height from them...... though that has its own stability issues.
Are the legs wood? If so you could just make longer ones from scratch. Otherwise, if you're up for something a bit more complicated, you might be able to use a finger joint to extend the legs. Matthias Wandel recently posted about repairing an axe handle where he used finger joints to extend the handle from the breaking point. If its strong enough for an axe handle, I have to imagine it would be strong enough for a stool leg.
Here's an image from the referenced article showing how the joint works
After gluing together, he shapes the new part to match the old part.
You could try a dowel screw and a short length of matching wood attached to each leg. I wouldn't use this method to raise up the stool more than 4-6" though, since the legs may become unstable with anything over that.