I got it working, using this code. This is THE RIGHT WAY to convert a euler into a Blitz3D quaternion!:
Procedure EulerAsQuat(pitch.f,yaw.f,roll.f) sp.f=Sine(yaw/2) cp.f=Cosine(yaw/2) sy.f=Sine(roll/2) cy.f=Cosine(roll/2) sr.f=Sine(pitch/2) cr.f=Cosine(pitch/2) vectorw.f=(cr*cp*cy-sr*sp*sy) vectorx.f=-(sr*cp*cy-cr*sp*sy) vectory.f=(cr*sp*cy+sr*cp*sy) vectorz.f=-(sr*sp*cy+cr*cp*sy) EndProcedure