It's always nice to work with a really good singer. But sometimes you have to work with the singer you're given. Especially if the singer is you!
There are really two questions here. A thin vocal sound isn't necessarily nasal, and a nasal tone isn't necessarily thin.Â
The root cause of a thin vocal sound is a singer who is inexperienced and untrained. Just like going to the gym builds up your muscles, singing builds up your vocal 'muscles'. The more you sing, the richer your voice will get. Training will accelerate this process.Â
A nasal tone of voice may be caused by a blocked nose, the result of cold or flu symptoms. There is no alternative than to wait for recovery. Sometimes there may be a more permanent medical problem for which qualified advice should be sought. Remember that an ear, nose and throat specialist will know a lot more than a doctor in general practice.
A nasal tone might also be caused by poor singing technique. People who learn French are sometimes advised to try and achieve a more nasal sound to their voice. Well if this is something that can be learned, then perhaps 'de-nasaling' can be learned too!
There are of course technical methods of thickening a vocal. But nothing works nearly as well as improving the sound at source.