When you eat LCHF and keep calories in mind, you don't have to lose weight. The goal of LCHF is to decrease your blood glucose and the required insulin response for dealing with the glucose. Staying low carb, you can increase your calories two ways:
Make sure you are eating enough protein. There are resources that can tell you what your protein should be for your height when combined with your activity level. Keep in mind that some of the protein is converted to glucose when metabolized so you don't want to go protein crazy, just make sure you are getting enough. Protein doesn't necessarily spike your blood sugar, but it can use insulin. In "Dr Bernstein's Diabetes Solution", he recommends increasing protein for weight gain. I think he did this after finding not all of his patients liked the higher fat recommendation.
The usual method for LCHF to increase calories and weight is to increase fat. The easiest way is to "bulletproof" your coffee or tea by adding oil and butter and then throwing it into a blender so the fats blend in like cream instead of floating on top. Another way is to eat fat bombs. There are recipes galore all over the internet. I haven't checked the recipe section here, but I know that Pinterest has a lot. Most are sweet, but there are also savory ones if that is what you like. There are also high fat smoothies and soups to make which are low carb. Personal favorite is cream of mushroom soup
I think this is what you were asking. Hope this helps.