Simple. Capture them at 45 rpm and convert the sound to the proper speed. If you have DAK software or something similar, it's a done deal to convert 78 down to regular speed. But if not, then one of my web buddies made the following suggestions:
Adjust your sample rate. Put the 78 RPM record on a player mechanically adjusted at 45 RPM. Check "Stereo" and "16 bits and make the "Sample rate" at 25442. Why 25442? Because: 44100 x 45 / 78 = 25442. (For a 80 RPM we would have: 44100 x 45 / 80 = 24806). If you record your 78rpm (or 80 rpm) at the speed of 45 rpm. The music will be too low in tone and too slow. Restablish the proper speed by: Edit / Adjust Sample Rate / Choose "41400". Save the file. It will be at the right speed and tone. Reading your 78 rpm thru an R.I.A.A. preamp at 45 rpm will establish a tonal balance that you would not have had if you had read it mechanically at 78 rpm.