You used to have to do it over the phone with an account rep, but you may now be able to do it in their interface?

[help.yahoo.com...]
[help.yahoo.com...]

I would recommend being a bit careful about doing this though. We had 4-5 accounts, and had them under a master login... when they restructured Panama one of our clients turned into the master login, saw a few of our accounts, and the questions started to roll in. Yahoo was unable to remedy this situation, saying that once the accounts are joined they cannot break them apart anymore. I think this was due to the Panama import, and may run smoother now.

If you have a rep I would call them.

Thanks Werty, for the info. checked with a friend from Yahoo, it "can only be done by sales rep".

Even when we get it done, it seems that we can't manage multiple accounts under one master account if these accounts target different markets.

"Typically, only one master account is needed to represent a business. However, a large operation spanning different markets could need multiple master accounts. Please note that users cannot be transferred from one master account to another."

We recently had this done, as we had 2 seperate accounts and needed to add 2 more. We have a total of 4 sub accounts within one master account. We had the help of our dedicated account mgr/rep to merge them into one account.