Yes, with a quality set of small files you can widen the nut and bridge channels, although it might be a good idea to let a pro do it.
One thing to watch out for, though, is whether you'll then be able to intonate the bass after the change.
On my 8-string a few of the bridge saddles are all the way to the rear, so if I went with heavier strings or even tuned it to drop-D there's no space for the needed adjustment.
You'll have to look at how much room you have to work with and make some educated guesses as to whether it'll work.
Also, 8's are running a bit more string tension than their typically 5-string neck width usually has to handle, so the trussrod had better be working perfectly to deal with heavier gauges.
You may have to go with a set that's floppier than you prefer to make it all work.
To sum up: it may work with a good setup and careful attention to details, but then again it might not. I strongly recommend consulting a good tech before starting this project.
Hope this helps.
Ibanez#588/8 String#43/Peavey's #1