Hello Marcin,
unfortunately this wouldn't be possible, in order to directly add a post you would need to use the Blog section inside WebSite X5:

https://guide.websitex5.com/en/support/solutions/articles/44000529449-come-creare-modificare-o-rimuovere-un-post-

Let us know if you need anything else.

Eric

***** Google Translation:

Witaj Marcinie,
niestety nie byłoby to możliwe, aby bezpośrednio dodać post, musiałbyś skorzystać z sekcji Blog w WebSite X5:

https://guide.websitex5.com/en/support/solutions/articles/44000529449-come-creare-modificare-o-rimuovere-un-post-

Daj nam znać, jeśli potrzebujesz czegoś jeszcze.

Eric