If you have already entered many of these eighth notes, for an after-the-fact solution, use the JW Change Plug-in > Note Entries > Beaming > both Beaming Modes (Beam to Previous /or Next), will do this.
Thank you so much for your responses. The 1 over 1+2 worked perfectly. I don't write in 6/4 very often, but I do occasionally. It would, however, be simplified if Finale would allow this without using the composite function by simply adding a dotted while note to the rebeam dialog so one could enter 12 as the number of beats and a dotted whole note as the beat duration.