Does MagicDraw allow to import current Java source code, alter the OO model and create modified source code?
Yes, it does. We call this "round-trip engineering". You can reverse your source code, make changes in the model and regenerate it back without losing already coded parts (e.g. method bodies). You must use the code engineering set (the MagicDraw name for a collection of classes for code engineering). If you only do framework generation (Ctrl+G), you will receive only the generated framework (all other information will be lost), so be accurate at this point.