If you want to use weight lifting to help you gain weight and you are a hardgainer, then you need to follow a program that will benefit you best.
Following a plan like a bodybuilder will make you stronger and you may gain a few pounds but that is about it.
Those work outs are for people that are more or less gifted in the muscle development department compared to a real hardgainer.
What you need to do is work out less, but with more intensity than you may be used to.
You need to lift weights in a way that will stimulate your growth hormones to help build your strength along with a few other things you must do.
Following a regular work out program will not do this for hardgainers.
As far as eating more food, this is one area you can do what many body builders do.
During the off season many body builders try to get their body weight up so they can be larger during the next competition season.
They can really add on the calories and the pounds during this time.
In fact they can sometimes take it to the extreme.
Bottom line is you must really eat a lot of food so you start to bulk up.
I want to say eat whatever you want but you will add extra fat to your weight also.
This is not a bad thing because you are trying to get bigger.
You can always work off the fat later when you get your weight where you want it, and improve your muscle definition.