Rudi, I do not know whether your Diners Club is having this promotion or not, but here in the US there are two promotions with Diners Club Rewards points:
1) Double miles to Delta on Feb 1999.
2) 50% bonus for HHonors sometime in spring 99.
Rather than transfer United miles to HHonors, I suggest looking at Diner Club points for the transfer. Here in the US best would be to transfer Club Reward points to Delta, and then transfer Delta to HHonors.