basically, I'd like to transfer some ME2-DLCs i bought for one BSN-account to an Origin-account, so I can manage both games from one account.
Is that possible?
The BSN account is an Origin account. If your intention is to merge two accounts, you should contact official customer support. The easiest way is to use the live chat feature in the Origin software's help section.
I don't work for EA.
It is possible that you are not able to see your DLC for ME2 in your Origin client even if you added your game to Origin. That often happens for older boxed copies of the game and it has to do with the fact that Origin was released after ME2. The Origin team is working on integrating more of the older games into Origin and we hope that it will be possible to see the DLC for all your games in Origin in the near future.
Until then you will have to manage the two accounts. But as fredvdp already mentioned: As long as you use the same login (= email and password) for Origin and the BSN your DLC should show up in the Origin client automatically once the Origin team is done.