If they are unprotected AAC files (extension = m4a) that you got from ripping songs off CDs, then the answer is yes, and easily.
If they are protected AAC files (extension = m4p) then the answer is no.
If they are movie files (.mp4), then the answer is yes, but it will take some doing.
I'll assume the first one. What you can do is go to iTunes menu -> Preferences -> Advanced pane -> Importing tab ->
Change the Import Using setting to MP3 Encoder.
Now close preferences.
Now, if you highlight a song, and click on the Advanced menu, you will see an option to convert it to MP3.
When you're done doing that, you can switch the default import type to AAC if you would like.