No problem at all.
Just pick a spare column you're unlikely to ever use (say, multi-engine night), and change the heading to "gliding P/UT", then you can add up any gliding time separately and not inadvertently add it incorrectly into your aeroplane / single engine piston totals.
I used to use a commercial logbook, it had columns for 3-axis and flexwing microlights, SEP, glider training, and a bunch of other things that were never in there when Pooleys sold it to me in 1989: dead easy with either tippex, or nowadays an office printer and some spray adhesive. [Eventually I just designed and had printed my own logbook, but that's a fairly extreme solution.]
Job done.
G