Is there any way to keep any of of the leds flashing to comply with european regulation for night flights? Or is there any green flashing led that can be added as accesory?

Night flying or even very low light flying is a no go for the Neo.

But if you insist.
VIFly has couple options for night time flying FAA compliant lights.  But.....
1. Are your manual flying skills at a level that will allow you to fly at night. Neo doesn't like under lighted conditions if flying N or S. Tends to go autonomously out of control.
2. Camera sensor limits. Goggle image might be so poor if anything that flying will not be possible.
3. Added weight of the lights would significantly effect the flight times.  Lights are marketed for the DJI Mini series and other larger quads.