NOTE : Choosing the import type on the preferences menu alters the option you see on the File > Convert menu, heres how to do t.
1) Open iTunes Preferences.
Windows: Choose Edit > Preferences.
Mac: Choose iTunes > Preferences.
2) Click the General button, then click the Import Settings button in the lower section of the window.
3) From the Import Using pop-up menu, choose the encoding format that you want to convert the song to. To save the settings, click OK.
4) Select one or more songs in your library that you want to convert.
5) Choose File > Create New Version, then Create [format] Version. The format shown is the one that you chose in step 3. For example, if you chose Apple Lossless Encoder, select Create Apple Lossless Version.
New song files appear in your library next to the original files.