quick question: is it possible to record sound simultaniously using on camera stereo microphone and ane chanel of XLR (ie. wireless mic.)

If I understand the question correctly ...
Yes you can record audio from on camera mic. to one channel & external wireless mic. to other channel.

If I understand the question correctly ...
Yes you can record audio from on camera mic. to one channel & external wireless mic. to other channel.

In this case, as I understand, the stereo on camera mic becomes mono (L+R) - is it right?

I record almost all of my interviews in mono.
Often I have a wireless into one channel and a boom mic that will pick up more room noise into the second channel, so I can use both channels later in the mix.
In my final mix, the voice is always mono, usually I output in surround sound.

yes, I want to use stereo mic on camera (working as mono on one chanel) and 1 xlr input on the other chanel - is that possible in EX1?

With Sony EX1 - YES :-)
At the back of camera, remember to select Audio IN to EXTernal for external microphone.