Without getting into a long discussion...strength does not equate to how much muscle you have. I have seen guys with medium build have more strength then a bigger built up guy.
You talk about putting on weight and then turning it into muscle....you can't build muscle from fat. I'm not putting down weight training or building yourself up...but you have to ask Why? We called it putting on armour, and it works with some people, but remember, the more you put on, the more you have to drag around in the ring....and every fighter has seen big muscle guys come on strong for the first few rounds and then wear out in the end. I have actually seen some guys get so big in the arms and shoulders, it had negative effects on their hooks and cross. There is a lot of information missing in why you want to put on weight...but my advice is to concentrate on boxing skills and movement...and the lighter you can be, the faster and more effective you can be. The comments given by others have a lot of merit and you need to think this out before you get into doing the wrong thing with you weight and muscle mass.