hi,
short question. Is there a way to import a texture with alpha(transparency) channel?
greets
No. The Texture palette only supports the Color channel.
You can import an alpha separately into the Alpha palette. For best results it should be a 16-bit grayscale PSD. You can then use that with the texture for a brush.
thx for your help…but another question…sorry 
is it possible to save the 2d background in 2d mode as an alpha channel?
if i export my 2d painted picture to psd file, i can´t find an alpha channel…
There’s no alpha channel in the export. But you can get one. Simply press Alpha>Grab Doc. Then export that alpha. You can open it in Photoshop and paste it into the appropriate channel for your exported color image.