You have to be root to add entropy to /dev/*random so you have to trust root - mind you many distros (ubuntu for example) use the CPU's RNG to provide external entropy by default, they trust Intel, you have to decide if you do too.

However kernel entropy is real, adding more entropy to the entropy pool is always is always adding more entropy. If the pool had 1000 bits in it and you add 5 bits from the kernel and a fake 1000 bits from a bogus external source it still has 1005 bits of entropy (however /dev/random thinks it will be able to extract 2005 bits)

We actually encourage our customers who are particularly paranoid to lie to the kernel about the quality of the entropy our RNG generates - saying the entropy quality is lower than it really is causes more good quality data to be mixed into the kernel pool (of course the defaults don't do this)

The kernel pools are actually quite small (4k bits) so there's a real practical limit to how much you can play this sort of game