I've red on Ironworks that it's impossible to import your main character from Baldur's Gate 2 ToB to Baldur's Gate 1 new game. But is there really no way? Even with mods?

you can import the character yes, he or she will have the name and basic stats but become a level 1 version of themselves ready to start out on their first big adventure ... something about a quick trip to the nearby Inn?

Ah, i was looking for a way to save the imported levels for a new game.

Although not exactly what you requested, you could enable the debug/cheat console, then use it to add an appropriate amount of experience to your imported characters. This would allow them to instantly level-up. If you have not applied any mods to remove the level cap, then your max experience will be quite low compared to Throne of Bhaal, and thus your maximum level will also be much lower.

Note that many encounters are not level-scaled, so even if you get this all to work, the game is likely to be much easier than intended. You may want to turn the difficulty slider up to compensate.

no the import system auto sets level 1 by design so your only option is to then hack the save file as advowson points out