You can use the MergeMarkups of SlicerMorph to merge open curve markups into a single markups node. Here is the tutorial Tutorials/MergeMarkups at main · SlicerMorph/Tutorials · GitHub
You should be able keep your fiducials in a single markup node without any problem. A fiducial markup node can contain any number of fiducials in it. Perhaps you are generating a Fiducial nodes as singletons (see this Markups node creation icon is confusing users). In any event, you can copy and paste fiducials across nodes.
