[MENTION=2240]Margo[/MENTION]
The plastic on the top of the refill will pull off.
Then get a standard cross black refill and pull the top off that and refit the plastic top from the original refill on to the new refill. 
You can also shorten the cross refills (if you don’t mind a little bit of mess
) with a dremil cutting disk to suit the shorter refill pens. Been there done that!