I have the same issue in that I have a 9 digit iberia number and can't transfer from AMEX MR to it. But, I really want to get at least 1 point into Iberia, here's why:

In order to transfer avios from BA, the Iberia terms say that there has to be a non-zero balance in the Iberia account.
I want the point to be in Iberia because the flight I want to book (BOS-BCN) is not showing up on BA as an option when using points (other Iberia flights do show up, but not this one).
I was going to try to buy a small number of Iberia points, but even buying points seems to have a restriction that there must be at least one point in the account.

Any thoughts on how to get my Iberia account to have points in it, so that I can then transfer AMEX MR -> BA Avios -> Iberia Avios