Intumescent Strips - Fire Seals Direct

You could screw them on, but the door would no longer be a fire door so it would be expensive

From your comments I am unsure whether you are referring to the hardwood door lipping or the intumescent strip?

The hardwood lipping should be prized away from the door leaf only by just enough so that Cascamite adhesive can be applied to the surfaces that are to be mated. Then use small panel pins to secure the lipping whilst the adhesive dries. You can then either remove the pins afterwards or you can bury the head of the pins into the hardwood lipping and fill over the heads. 

A loose intumescent strip can be secured to the edge of the door or the door frame reveal using small panel pins. https://www.ifsa.org.uk/