Thought: There isn't any way to convert waystone/sharestone/portstones to each other.

Suggestion: A crafting recipe. Putting one of the blocks in returns the warp-stone. This can then be used in another block or kept for teleporting.

This would probably be best alongside the shatterable village waystones to reduce incentive to steal them.
Thanks 🙂

I don't think there is need for this in the official recipes. Conversion recipes are the exception, not the rule in both Vanilla and Modded Minecraft, and I can't think of a reason why Way/share/portstones in particular should be convertible.

However, there's always the options to make recipe additions yourself using datapacks or mods like CraftTweaker.