So I am underweight, want to gain some mass, mainly in muscle and I know that resistance exercise + proper diet are the key here. But also I would like to do some yoga ( 2-3 times a week ) because I need to improve my posture, flexibility and work on stress relieving. It's of course very important to let your body recover after strenght training and many things can suppress or slow down this process ( for example running in my case would suppress satisfactory weight gains ). On the other hand I heard that working on flexibility can even contribute to muscle gains. So I was wondering how does yoga fit in here? Should I resign, at least for now, from it? I am talking about the standard hatha yoga without any cardio or heating.
I would think keeping up the yoga would help with recovery, keep you active and help you put more into your weight training. There's a limit to how often you can do resistance work, and it's hard to eat the amount of quality food you probably need if you sit around the rest of the week. Personally my recovery days have swimming and yoga - the swim is gentle but allows me to stretch my muscles, get a full range of movement and gets the blood pumping through the muscles with all those nutrients. It might cost me 100 calories or so, but I can easily make that up through increased appetite from just being in the water. And I'll sleep better too since I have the yoga in the evening, stress reduction is just as important for recovery and growth.
So I would definitely think you would be better off all round to keep doing your yoga.
Do you want to NOT do cardio, or because you've read that chronic cardio is bad you think that you shouldn't do it?
Yoga is great for your flexibility and your posture of course so do that if you'd like. But it's not going to add serious mass or burn any significant amount of fat. However, if you take certain poses and use them for stretches after your weight training (never stretch a cold muscle), they will help your weight training be more productive by helping reduce soreness slightly and keep you more mobile the next day or two.
My business is designing nutrition plans and exercise regimens for individuals based on their own ability and fitness status, their goals, their current capacity for exercise, etc... I use an ancestral approach for dieting be it fat loss, muscle gains, strength gains, whatever it is the client is looking for.
In your case, wanting to build muscle I'd suggest you adopt a good ancestral diet with what people have called safe starches (tubers), and fruit as a part of it one or two days a week (the day before and the day of your heaviest weight training session) and a regular ancestral diet...I don't care if you do paleo, primal, the ancestral anabolic approach, whatever you like.
Training, I like Starr's 5x5. Most of my regimens I create are based on this, adapted of course to the client's ability. The basics are Monday, Wednesday, and Friday you lift, doing 5 sets of 5 repetitions, utilizing the squat, bench press, dead lift, barbell row, chin up, and overhead press (or a variation of them if you can't do the actual lifts named). On Tuesday and Thursday do sprints (8 of them 60 meters each). Sprints boost GH and help you grow better, allowing better muscle gains, where distance running catabolizes muscle away. Walk every day because it's just good for you.
Yoga should only help with recovery. If you want to gain mass, keep reps low, and heavy. Also do some sprint intervals, as it does improve HGH. I would lift 2x per week and do sprints 2x per week. do yoga on other days. Eat a lot, and keep your protein to at minimum 1g per lb bodyweight.
Yoga is a great form of active recovery. It will help increase blood flow (and therefor nutrient delivery) to your previously worked muscles. Not only that, but it will help elongate your muscles and give you literally leaner gains. All the while reducing stress and cortisol, which will contribute to more desirable gains (i.e. gains in all the right places). I personally take a yoga class at my gym about 1-2x per week, usually on weekend morning. Keep at it bud.