I bought a Nexx WT3020F and I'd like use it in this way: connect the device to my network by WIFI (no LAN cable) then share HDD/USB pen drive content by samba with all device of my network.
By defualt I think this is not possible with standard firmware.
Would it be possible with OpenWRT?
After flash OpenWRT is there a way to return to official firmware?
For future you probably want to use the ideal hardware page, and then buy something. https://openwrt.org/supported_devices
But you seem to be in luck , there is a page for that device, https://openwrt.org/toh/nexx/wt3020
but be sure to read it through- there seem to be many versions of the wt3020 , with some having different uboot and may get bricked. And others where the failsafe recovery won’t work either .
For that device, it may be a bit underpowered for what you want to do, but give it a try 
. You’ll be limited with space as it’s only got 8mb of flash space, but if you do run out search the wiki - some clever people have found ways to install larger modules into ram space 
(so you’d need to reinstall whenever your device reboots)
So with 8MB I should reinstall openwrt every device reboot?
No, 8 MB flash are still comfortable for 'normal' purposes (including some advanced features) for several years to come. 4 MB flash however are no longer sufficient - and 64 MB RAM are enough at the moment, but leaving little headroom for the future.
Yes, personally I'd look for more than that for buying new devices, but that's merely to keep the options open for the future (e.g. packages like samba4 require at the very least 16 MB flash, so do installations containing asterisk or similar rather advanced features), but 8 MB flash aren't really limiting at the moment or for the forseeable future.
OK for future updates 8MB could become a limit (if I understood correctly).
However I have read this: https://openwrt.org/docs/guide-user/services/nas/usb-storage-samba-webinterface
So for my use (NAS via WIFI to share USB content) I think I should install:
I have check the "installed size" of https://openwrt.org/packages/table/start and I don't think but I would like to be sure (in the past I had a bad experience with a Nexx 1520 with only 4MB but the 3020 have 8MB).
Also I would like to be sure I can connect the "NAS" via wifi (so not cable) to my network. It seems obvious but if someone can confirm I thank him.
Thank you all.