I like to use kmymoney for a small company that is just starting out.
I have not managed to find a good way to enter N shares of treasury stock to kmymoney and how to enter the original share capital (shareholder equity)

Thank you for your detailed instructions.

Looks like it's better to keep the treasury stock off the books.
Question "how to enter the original share capital (shareholder equity)" remains.

You can enter a transaction and use the type "Add shares" to enter the initial balance of your stocks.

Thank you Hei Ku.
Now, what about the share capital, that is a portion of a company's equity that has been obtained (or will be obtained) by trading stock to a shareholder for cash or an equivalent item of capital value.

Do I need to create special accounts? Special transactions or categories?

Thank you for your help.

You have created an investment account, have you?
All stocks, shares, etc in that financial institution, go into that account.
If you have cash money, you have to create a brokerage account at the same time.

In the case of a company equity, perhaps that's better to keep in a separate asset account. Unless you want to treat as just another share (that is what shares are, after all), but you don't sound like it.