I would like to add existing item to all shops. How make it shortest and easily way ? Better via tes4edit then CS if possible.

A number of questions:

Do you really mean "all shops", or just vendors of a certain type of goods?
Does the item have to be available always, or randomly?

The answer is probably going to be adding the item to a leveled list with OBSE's AddToLeveledList function, and those questions are going to determine which lists are appropriate, and what the parameters are.

1. to all shops
2. always

There is no single list that covers all vendors' inventories, so you'll have a lot of work to do. The "always" part also makes it harder to add the item, as it can't be put in a list that the game will randomly select from.

Whatever method you choose needs to consider compatibility with other mods. If you add your item directly to every vendor's merchant chest, any other mod that does so will undo yours. Diito for a direct addition to the NPC's inventory, So the "safe" methods are to add an additional vendor container, or use a script with AddToLeveledList to do it. Either way, it's going to be a lot of work to achieve what you want.

Directly I want to get some mod working for Nehrim. Some item must be easy to get by PC, but ... not too easy. Maybe I should find other way.
Thank you.

One method would be to create a new chest and add the desired item(s) to it. You could then place that chest in each vendor's area. They would then sell the item(s) contained in the chest and there would be no conflict with other mods. This method would require the use of the CS, though.