No. Although you can add amounts to most unit-linked policies, the life assurance company will separate the original amount from the new €50,000 and treat them separately for tax purposes.
That's not correct. In most cases the life assurance company can add the new amount to the existing policy, and there is a tax benefit for the investor if the policy increases in value.
To use the figures quoted, after the new money is invested, the value of the policy (ignoring charges) is €60,000. Assume that the investor encashes the policy at a value of €90,000 (i.e. the value has grown by 50% - this may have taken several years but the time doesn't matter much*). Then the tax will be calculated on an overall gain of the final value of €90,000 minus the total invested of €70,000, i.e. €20,000.
If the €50,000 had been invested in a separate policy which had gained 50%, then the taxable gain on this policy would be €75,000 - €50,000 = €25,000. The same 50% gain only brings the value of the first policy to €15,000, so there is still a loss, which can't be offset in this case.
So there is a definite tax advantage in adding to a policy where you have accumulated a loss.
*Things get more complicated if the policy is in force for more than 8 years and you have to take into account the tax payable at that time, but it shouldn't affect the main point.