Yes it is possible and no you don’t have to combine your proteins. Your body will do that for you. Increasing muscle mass, besides having the right strength routine, will involve eating a ton more food period. There are plenty of vegan and vegetarian sources of protein as well as bodybuilders and other athletes who are vegan or vegetarian. Stores like The Vitamin Shoppe and health food stores are pretty good about keeping protein supplements in stock that have no whey or casein. Once you start looking, you will find that there are more options than you can possibly imagine.