Is it possible to add this modified MiG-21 to official flightgear hangar so others can download it.
Yes - usually contributions like this go through the maintainer of the aircraft you're contributing to - so you could e.g. ask on the mailing list whether the person who takes care of the plane on FGAddon is currently around.
If no one steps forward for a while the maintainer can be considered absent, someone else who has commit rights to FGAddon can review the patch and commit it on your behalf.
Generally you can expect to be asked licensing questions (where is the content from,...), though in this particular case where it is a copy from another craft, this doesn't look problematic.
There's also 3rd party development (i.e. planes which reside on other repositories and are not regularly merged to the official hangar - like what PINTO linked) - generally the FG project does not decide upon how things are contributed there, you have to get in touch with the 3rd party hangar admins.