No, the compressed files will be converted to uncompressed PCM when you import them into your project. You're therefore splitting uncompressed wave files. Of course, at that point you'll have already suffered whatever degradation the original encoding imposed; the wave files won't sound any different just because they're waves. When you re-encode the second time, you'll add your own degradation in the process. However, if the original file was high-quality and you use high-quality settings to encode the final product, the result can be quite transparent.
AVI files
might work better, but they're not necessarily uncompressed. They can be either compressed or uncompressed. AVI is just a container, so the audio within can be any format.