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Hello Kamil,
thank you for coming to the forum.
From my check the feature you are requesting is not available. So you cannot import directly into the planner from Excel.
But looking around I found some links which I will share below of how some people have found a workaround using Power Automate.
https://tomriha.com/how-to-import-tasks-from-excel-into-planner-with-power-automate/
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
If you are having any issues, do post a reply in the power automate forum: https://powerusers.microsoft.com/t5/Microsoft-Power-Automate/ct-p/MPACommunity
Share an update when you have one
Best Regards Fuad