This is a bit confusing. Are you trying to force Weinmann pads into Universal holders, or are those Weinmann holders that you've put on to Universal brakes? If the former, it is not clear that they will fit at all because the pads are different sizes. If the latter, then you can generally press in the pads using a vice or woodworking clamps, sometimes helped a little by smearing some dishwashing soap on the pads.

That said, if you are actually going to ride this bike I would highly recommend that you NOT use NOS pads. Brake pads degrade as time goes on simply from exposure to the atmosphere and sunlight. Instead, go to the Kool Stop International - High Performance Bicycle Brake Pads Since 1977 site and order some fresh pads. They have both Weinmann and Universal reproduction pads.