Sure a trainer can be a glider. 2 problems though. The typical trainer has a heavier wing loading than the typical glider. This means it must go faster to fly, so hand launching will be tougher. Also the typical trainer has a bunch more drag than the typical glider, so it doesn’t glide nearly as efficiently. You could make a semi decent slope soaring glider from a trainer wing by building a light weight glider style fuselage and tail for it. But the wing will still not be very good for thermal flying in lighter lift. The wing can be improved a little by extending the span. But if you are going to take the time to do all this, you might as well just get a glider kit and build that. There are some ARF gliders available.

Multiflyer