This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Yes. The cheap balsa "snap together" toy gliders (i.e. North
Pacific) are *NOT* strong enough for flight conversion; their
wings are too thin and will shread. Plans for Jetex models are
usually too flimsy for model rocket power [I've estimated that
a Jetex motor is about an A 0.2-P in the NAR system].
However many HLG kits and plans are convertible. Also look at
the Catapult gliders, essentially HLGs flown with a fixed size
rubber band instead of arm strength. A wealth of HLG plans are
available from the Academy of Model Aeronautics (AMA),
National Free Flight Society (NFFS), Zaic yearbooks, and some
of the other RC modeling magazines. I highly recommend the
NFFS newsletter, journals, plans, and publications as sources
of free flight glider information.
Usually, all you need to do is to add a pop pod (or a fixed
pod if you want to keep things simple) to the HLG, and perhaps
invert the rudder. The references at the end of this part of
the FAQ list several good HLG plans.