You have a couple of options: you can use a small amount of a similar wine, displace the volume in the carboy with sanitized marbles or transfer the wine into various vessels that will accommodate the volume with minimal airspace.
i have been told by a great winemaker that if you dont have a similar wine to top off with a bottle of chardonay will not affect the flavor of most wines... many winerys that dont even sell chardonay will make it for topping off of other wines..... again i dont personally know how well this works but he told me that he does it all the time and it works great
I agree with Wade... the Chardonnay grape itself is usually pretty free of any real strong personality, (with the exception of very high quality examples from very low yields) but there are lots of Chards out there that are buttery and heavily oaked and that would be best to avoid for this purpose.
However, unless you have alot of space to fill, the chance that the topping wine will have much affect on the end result of the final wine is unlikely. I learned this lesson a few years ago when I had a rather thin and disappointing kit wine in a carboy that needed about 1.5L of topping wine, which is more than you're likely to need. After adding a heavy, rich wine hoping to add some deeper flavour and body to the kit, I tasted the wine and could perceive no difference despite the large amount... ultimately, even adding 1.5L amounts to a tiny percentage of the final wine and therefore didn't affect the aromas, flavour or body. As long as you're adding wine and not water, you'll probably be alright.
If you make a fair amount of wine, it's worth pouring the lees and dirty wine in the bottom of the carboy after racking into an appropriate sized bottle and allowing it to settle. Once you have a bunch of these bottles of dregs settled out, you can siphon the clear wine off the top into a single "master" bottle and use this as topping wine. Just be careful before you add any wine from these bottles of sludge to your master bottle of topping wine that they haven't developed any nasty aromas/flavours from all the lees contact.