Mageia forum

is it possible to have some lines always at the top, not dropping down in the list as time goes by, on Klipper?

Nope.

well
it's a pity

You're asking the question at the wrong place, try https://forum.kde.org/ or suggest it at https://bugs.kde.org/ as enhancement request for KF5.
But klipper is just a clipboard (history) manager so it doesn't make sense to add any persistent entries there. Use something that allows you to add text templates/clippings.

As you don't mention what you need this for can't help you further.

for thing as simple as an email adress that i write here and there 5 or 6 times a day ...

Bind it to a hotkey combination.