There is no need to import the seed anymore now that you have transferred the founds.
When you create a walled you receive a seed that you have to write down and keep it somewhere safe. That seed is actually how you access your wallet, it does not matter if it is nault, nalli nanowallet, natrium or any other. The same seed you can import anywhere and access your wallet.
For natrium, in the menu you have the option "Load from paper wallet". Or, first time you open it, it asks you if you want to create a new wallet or import one. If you already have one, it does not matter where, you can simply import/write that seed.