Is it possible to use BMX steerer (stem+handlebars) on fork on folding bicycle? II want to replace my folding bike's steerer with a BMX setup.

I've been told by a bike shop that it is not possible because the steerer tube protrudes 10 cm higher than the frame tube.
There may be a compatibility problem with the BMX stem with the top bearing of the headset that would prevent it clamping the headset properly, however, I strongly suspect that the BMX bars would be far to low to be safe, and that's why thw bike mechanics wont touch it.