Thanks to all for your suggestions!
Unfortunately it seems that none of them can work...
I emailed Diamond Desk @ Hilton, asking them to remove 3 first digits from my BMI number and try to add it like this. Got reply that them tried, but the the HH system was not accepting it.
Then them tried to remove LAST 3 digits - and in this way them system accepted the number! I think that this is a joke - removing LAST 3 digits my BMI number is almost entirely comoposed of zeros

. So I have HUGE doubts this will work.
So if somebody have any other suggestions - I'm open to hear them

.
An additional question : have somebody tried to use 11-digit BMI number with United already ? I have tried to add it when booking a flight on United site today - it was not recognized