Yes- you can use the same recipe for HP that you would use for CP. The only difference is that you will be heating/cooking it to saponification. Oh, and depending on how much water you normally use for your CP recipe, you may need to add more for HP. When I HP one of my CP recipes, I just make sure to use a 'full water' amount, which is basically the same as using SoapCalc's 38% water as per oils amount. If you don't use enough water, your soap batter will be too dry to work with (because of evaporation from the applied heat).
In my experience, adding liquid during the cook is difficult for two reasons. One, it takes a while to mix in during the cook. Two, it's an extra measurement to remember to make notes of.
I have found that using the full amount of water to start with- 38%- works very well for me and the results are very consistent.
Yes, you can, but as Stacy said, it won't be as easy to incorporate as compared to when adding it up front, but it's still quite do-able nevertheless. Whenever I ever find my HP batter running a bit dry for whatever reason, I like to spray it with some water from a spray bottle. Spraying water it as opposed to pouring it in helps to keep me from over-watering it.